A guide suggesting which skills or talents are better suited for each position, based on my experience in the game so far. It's not a "must have these skills" guide, it's simply a helping tool for anyone with doubts over which skills to train and where to train them.
Starting from the back to the front, here comes the Goalkeeper. He is probably the most important defensive player, and a good goalkeeper will get you more points than a (few) good defender(s) or midfielder(s).
All goalkeepers can train 2 skills, as it's considered an Universal class, so make the most out of it. You should at least have 1 defensive talent, as simply having a high Defence Attribute will only get you so far. The 3 Defensive talents you can get as a GK are Playing Out, Strong Goalkeeper and Reaction.
Playing Out will stop Olympic Kicks, which is always useful, but also gives a bonus to Control (25 per level) against Crosses, which is very useful (more than stopping Olympic Kicks even, as I haven't seen many opponents actually use them against me, despite having good chances to use them, plus having defenders in the pathway will also roll against it).
Strong Goalkeeper stops Cannon Shots, which is great particularly if you don't play or want to play a Sweeper, the physical counter to Cannon Shots.
Finally, the Reaction stops Rainbow Feints, in my opinion the most devastating skill in the game, as a Rainbow Feint 3 Forward will finesse by your entire defensive setup. It's not a must have, to stop Rainbow Feint forwards, there are other ways, but it's always great to have.
After getting one of these, you should consider if you need another. If you think that he doesn't need another, then consider having Long Pass, as your GK will probably not have a high Passing Attribute (after all, Defence and Control are far more important to a GK), and it allows you to pass directly to your Midfield Players in one move (or zero, if you roll successfully against an opponent in the way), starting a counter attack.
Next up, comes the Sweeper, probably your Nemesis as you try to score goals when you're the underdog. He moves automatically closer to where the ball is everytime it's on their half, which means that if you have a Cannon Shot player and you're trying to get in position, he will get in your way unless you can stop him.
Considering his ability, he has the drawback of moving closer to a player that has Rainbow Feint, which will help him getting in front of your GK, if he can beat your Sweeper that is. Because of this, Reaction is a very useful talent to have, specially if your GK does NOT have it.
Otherwise, like the GK, Long Pass is a nice alternative, for the same advantages that I explained on the GK section.
Head Play is also viable, as he will be usually the first defender to be able to head it away, if they cross from the outer tiles.
Finally, Slide Tackle, as for most Positions, is also a good choice, although not as good for Sweeper as it is for other defenders.
Remember, only Universal classes have 2 skills to train, Sweeper (and all others that don't have Universal in the name) is not Universal, so he only gets one, so take your time to decide. In my opinion, the first 2 I mentioned are the most important to have on a Sweeper, for what it's worth.
Central Defender (CD)
The Central Defenders are your first line of defenders, as they will automatically tackle every opponent that moves into their vicinity (the 8 tiles around him). Beware that they do NOT tackle automatically if an opponent simply receives the ball, they have to move to trigger it.
They also only tackle automatically if they are positioned inside the 3x3 tile box in front of their goal, so you shouldn't have them move outside of it often (they can tackle automatically moving opponents that are positioned outside of that box, but then you have to move them back inside). He also will not be able to use this ability if an opponent uses a skill or even a normal move (like Dribble) against the CD.
Because of this, Slide Tackle is not as useful as it should be (just like for Sweepers), but it's not useless as well. Central Defenders can only move one tile with or without the ball, so getting near a forward a few tiles away with a CD to use the Slide Tackle isn't the best use of his skills. But if a forward has the ball near your CD, it's a nice way to get it back.
On the other hand, Head Play is far more useful to have on a CD than on a Sweeper, as he will also challenge any opponent going in for a header. If you buff him up on Control, it makes a Head Play CD a nice weapon to use on an attacking Corner.
Like I explained before, he can't use his ability if he gets beaten by a skill, and Rainbow Feint is one of those skills. So, yet again, Reaction is a great choice for CDs.
Interception also helps against the Passing skills (Layout Pass and Long Pass) from being used against your CDs, so it's worth some consideration.
Long Pass or Layout Pass are always good, but not so much for CDs, as they can only have one skill, the other ones I've mentioned are much more useful than Passing skills.
Universal Defender (DF)
The OG defender, at least from before Update 10 which introduced all these specialized positions, the Universal Defender can intercept nearby passes and shots on goal which neither CDs or Sweepers can. Other than that, he's a vanilla defender that can play on any position out back and is solid on his endeavours.
He does get 2 skills to train, so let's pick a few that suits them. For starters, Slide Tackle is a good one to choose, as since you probably haven't picked it for CDs or Sweepers, if you have been following my advice so far. DFs still only can move 1 tile, but since he can get another skill, it's not a bad choice to have him in between 2 other defenders.
Head Play is also nice, but I would rather give it to a CD than a DF, like I mentioned on the previous section. With the ability to intercept nearby passes and shots, DFs can easily go out of position quite frequently, so Head Play will not be as useful for DFs.
Long Pass or Layout Pass are good ones to have as well, and more useful for DFs than Head Play, in my opinion.
Reaction is also always good to have on a Defender, but if you have a mix of DFs and CDs, and those CDs have Reaction, you can bait attackers to use the skill against your DFs, and then your CDs can tackle the attackers to win back the ball. So Reaction is not as essential on a DF.
Intercept, on the other hand, can be also considered, maybe as a second skill for a DF.
Remember, 2 skills to train, so there's plenty of combinations to try.
Full Backs (LB/RB)
For the final defensive position, we have the Full Backs, the 2nd position that can move the most tiles with and without the ball. They can also lower the skills potential roll of opponents, be it for or against them, whenever they are near them AND in their prefered position (the 2 outer collumns of tiles on their side of the pitch, so they can also be useful to lower the opponent's defenders rolls to help your attackers do a skill against them).
With that in mind, Slide Tackle is a very useful skill to have on a Full Back, as they can easily move near an opponent that is Holding the ball far away, so they can't be cooking up an attacking move (in my experience, sometimes the other team like to hold the ball in the midfeld just to annoy me). You can also use it to do the same against them, with the help of the 3 tiles to move in one go, you can get away from other players and hold it, forcing the other team to waste moves to get near your FB again.
Long Pass is, again, a nice one to have, as you can use the extra mobility to get in a position to get an extra move out of it, at the very least.
When I mention Long Pass on this guide, you can also (usually) consider Layout Pass with it, just know that it has a much smaller range, as it's only able to use for the same range as a normal Pass. But if you can't get Long Pass, Layout Pass isn't a bad alternative, just have that in mind.
Reaction is also viable as well, specially considering that the FBs can reduce opponents' skill rolls. They don't reduce it when it's AGAINST the FB himself though (nor if the FB is using a skill himself against another player, he would be way overpowered), so that's why Reaction is more useful, as then a Rainbow Feint attacker can't use it against the FB, and if he wants to use against another defender, the FB will lower his roll.
Long Throw, I haven't talked about it yet, it's not a overly useful skill to have, at least in my opinion, I don't normally get many throw ins myself, but if you want to have it, then the FB is one to consider having it, as if he has to throw it in up in the pitch, if needed he can easily come back to defend.
Defensive Midfielder (DM)
The one covering your midfielders' back, the Defensive Midfielder is like an Universal Defender on steroids, as he can intercept any pass passing near him in a 2 tile radius. Also doubles his Defence attribute against any pass that goes through his tile. Finally, he gets one level of the Intercept talent, one of two positions that get a talent for free upon specialization.
So, which skills complements his role?
First of all, Slide Tackle. It's a great skill to have on plenty of positions, and DM is no different. It's not necessary to have it, specially if you pair him with a Central Midfielder, which can help tackle opponents. But not a bad option.
Long Pass is also a great choice, as, from his position, he can send it straight to a forward for an easy counterattack. Layout Pass doesn't have the same range, but it's still a great option if you have Wide Midfielders in particular.
Reaction starts to lose importance from now on, but on a DM is still somewhat useful, specially if you drop him into the defensive setup to help out your boys at the back. Not as necessary, but an option nonetheless.
Olympic Kick seems like an odd choice, but the fact is that none of these options are actually essential for a DM, so why not? I have it on my main DM, and he has a tonne of goals, guess from where? If you're going to have one player with Olympic Kick, there aren't many positions that can afford to "lose" one skill slot for it (well, it's not losing a skill, as this skill can get you plenty of goals against beast goalkeepers that don't have the Playing Out talent). If you miss, you will get your DM stuck up at the front though, so it's not without it's faults. But at level 3 you won't be missing many Olympic Kicks.
Intercept is also another option, he does get one level simply by being a DM, but that won't stop level 2 or 3 Layout Passes or Long Passes. I'm not sure if you get another slot if you already have Interception trained, I saw an opponent DM with Interception level 3 and no more skills, but that doesn't tell everything. If I get the option to test this out (or find out if it's possible or not), I'll update this.
Centre Midfielder (CM)
Perched at the core of your team, the Centre Midfielder is, probably, the main outlet of you team, but other than getting a small bonus to his passing attribute when in his position (a 8x3 box centred at the middle of the pitch), he also can move 2 tiles without the ball, and can Press opponents for free (hence why I said back there that if you pair a DM with a CM, you don't have to get Slide Tackle on your DM, as you can press with your CM and then Tackle with the DM with increased probabilities).
With this in mind, we get some options for his sole skill slot, starting with some attacking options, like Layout Pass or Long Pass. Even though he gets a bonus to his Passing, the added move that can be attained from one of these skills, make them a good candidate to be his skill.
Cannon Shot starts to become relevant from now on, but it's not exactly the best option for a CM, as he doesn't have much mobility to use it properly.
Same with Rainbow Feint, it's time to start considering it, but not exactly the best for a CM.
On the other hand, Slide Tackle is not a bad option, you can Press for free, but even better is to take the ball away for free, right? Specially better if your CM has high enough Passing, so you won't exactly need any of the Passing skills, when you can loft it to your striker accuratly enough.
Interception is also nice to have to stop Long Passes, which seem to be always successful against my midfielders.
Universal Midfielder (MF)
The most Universal of them all, the Universal Midfielder is a very versatile player, that can move 2 tiles with and without the ball, anywhere on the pitch. So you need a makeshift Forward? He can get behind Defender lines easily. Need some help to take out the ball from an Holding attacker? He can get there faster than a CD or a DM. He doesn't have any other advantages, other than having 2 skill slots.
So, you can pretty much have any skill that you want on him.
Some skills can take a lot of advantage of his higher mobility, like Layout Pass, Slide Tackle or Cannon Shot, as you can get in position with the ball or near a forward or where the ball is and get it back in great style.
Nutmeg is in general a very situational skill, but as you can move 2 tiles a time with the ball, you get by a pesky opponent, zoom away from him afterwards and get a move for free in return. Not a bad option.
Olympic Kick is also a good option here, as you can choose another skill to go with it for your MF and not feel like you've wasted a slot.
Rainbow Feint is slightly more useful here than on a CM, as you can also get near an opponent faster than with the CM.
Intercept is also an option, as you can position him in the way of any player with Layout Pass or Long Pass.
Long Throw is also, like Olympic kick, an useful option for put on a MF, as you can still have another skill. On the other hand, if you already have it on the Full Back for example, you can have Head Play instead, to win those throws more easily, and maybe score from crosses as well.
Wide Midfielders (LM/RM)
The kings of mobility, the Wide Midfielders can travel a whopping 4(!) tiles at a time with and without the ball. He can also Press an opponent for free (just like the CM). Both can only be done when he is on the 2 outer collumns of tiles on his side (just like the Full Backs, same collumns).
With this in mind, what can we add to his repertoire?
What about Cannon Shot? With his quickness, he can get in a position to use the skill in ONE move. The problem is when the other team has a Sweeper, as he will get in the way if left unbothered. My main way to stop him is to leave a Forward in the way, so he won't have space to move. This requires some planning, either waste a turn to prepare the shot and then execute it on the next turn, or use Long Pass or Layout Pass successfully against an opponent to gain an extra move, or just be lucky, or all three combined.
Rainbow Feint is useful, as you can either use it to go past another midfielder in the way, or even use it to try and score. If you only want to be able to go past players in the way to cross, then Nutmeg is a great alternative, as it doesn't have any counter other than getting a smaller roll than your opponent. And needs to have space behing him, but at the sides normally you won't have to worry about it.
Long Pass and Layout Pass are also some good choices, allowing you to put your strikers closer to goal.
Head Play is not a poor choice, specially if you also have another player with Long Throw, as you can position him in a crossing spot and win the challenge for the throw in, and if needed, help out on a corner.
Slide Tackle is also a great option, as you can get near anyone pretty fast, and also allows to take the ball away from a pesky Defender with high control, and maybe create another chance for a goal.
Attacking Midfielder (AM)
Sitting behind your forwards is the Attacking Midfielder, along with a neat free move that is the False Kick, which allows you to move an opposing defender out of his position and closer to you. He also can shoot with a reduced range penalty, compared to a forward, and intercepting defenders have a 60% reduction against his passes. I don't normally use AMs but they aren't bad at all.
With this in mind, the skill slot comes to play.
Layout Pass for once is a great option, perhaps better than Long Pass, as you will normally be always in range to use it, and can use it put a forward behind their defensive line. And you can couple it with the False Kick to create the hole for your Forward to get in. The only problem is if they have CDs with higher defence than your forwards' control, then you should see if you use the Skill against said CD. You should know what to pick if you get both options to train. Remember that Long Pass only works at a distance of 3 to 5 tiles, so if your Forward is too close, you can't use it.
Slide Tackle is yet again a great option, specially if a DM intercepts a pass or a CD takes the ball away from your forward.
Head Play is not necessarily a good choice, but if you don't have a forward with this talent, could be a nice complement to your attacking lineup.
Cannon Shot is also not the best here, as he doesn't have much mobility to get in position unless you can create a hole for him.
Rainbow Feint on the other hand can be more useful, even more if you don't have a forward with that talent already. With the False Kick you can pull a defender closer to you, and then use the Feint against him, and then against other defenders until you get to the GK, all this without using one move.
Universal Forward (FW)
We're now on the final stretch, and now we are going to focus on attackers, starting with the Universal Forward. This guy can move 2 tiles without the ball (only 1 tile with it), can make a move again after an unsucessful shot on goal (if you still have moves to make in your turn), when he gets Competitive Rage he remains in it for one extra move, and finally, gets a 30% increase in Control when he has the ball and it's the other team's turn to move.
Oh, he also get's 2 skill slots. Neat, right?
Knowing this, not having Slide Tackle is borderline criminal. I mean, why not? You can take the ball from a defender, get one more move in return, and still have another slot to use another skill to try to score. Since the FWs can move again after a blocked shot, if a defender blocks your shot, you can use this skill to take the ball again, and probably get even closer to the goal. Tactics, right?
Rainbow Feint or Cannon Shot start being super relevant now, so the other slot will probably have either of these one (you can even have both if you want to, just to make sure you WILL score in some way or another).
Olympic Kick can be a good consideration, again because he has 2 slots, so if you don't have it yet on another player...
Nutmeg is also a nice way to get by a defender and put you in front of the keeper, if you can evade their Sweeper (if they have one).
Head Play is also a good option to have on an attacker, specially on a FW.
Your focus from now on is going to be to score goals, so Rainbow Feint or Cannon Shot should be in your first thoughts. But the other skills aren't to be discarded, as they can get you closer to the goal. Even Slide Tackle, a defensive skill by nature, can probably help you get your goal.
Centre Forward (CF)
The tip of the spear, the Centre Forward is going to be your guy playing on the edge of the other team's defenders, as he has the automatic ability to move a tile closer toward goal at the end of a turn, provided that there is some space for him to move. He also is the 2nd and final position to get a free skill upon specialization, which is Level 1 Head Play.
With this in mind, you kind of need to have players with Long Pass to take advantage of his automatic ability, as he will move outside of a midfielders' Layout Pass range very easily. Just some warning before you pick a player to be CF and you don't have Long Pass in your ranks.
For the CF himself, Rainbow Feint or Cannon Shot, again, are the best options, as you will probably be right infront of the GK after some turns, so you can use it without complications, provided you can feed your CF, like I explained before.
Slide Tackle is not bad, but since you can only have one skill and a CF only moves 1 tile at a time always, it's not exactly brilliant.
Just like with the DM and his free talent, I don't know if you can train your CF to get Head Play up to level 3 and still have another slot for another skill. In this case he doesn't really need level 3 Head Play, I would rather give it to a FW.
Nutmeg is a good option to have, because the CF needs a tile to be open to move to, and sometimes it's just not possible if the other team has stacked up on defenders against you and they don't get displaced easily. But Rainbow Feint does the same trick, along with being able to use it against the GK, so only pick it if Rainbow Feint is not possible to train.
Wide Forwards (LF/RF)
Finally, we get the Wide Forwards. These guys have a neat little free move called Break In, which can be used at the corner tile on their side. It allows them to get into the penalty area, challenging every defender in the way with a 50% penalty to their defence vs. your LF/RF's full Control. Any CD that can challenge you on the ending tile (i.e. when you move into their vicinity) will get that penalty as well. He can also move 2 tile with or without the ball, but only when on their prefered position (the same as the other wide players, the Full Backs and the Wide Midfielders).
With this in mind, the skills that are relevant to him, are pretty much the same as for the CF, as the LF/RF will also be right in the GK's face when use correctly. So Rainbow Feint and Cannon Shot are no brainers. Just remember, you can use the Break In right from the kickoff, but you can't use any skills on the first turn after kickoff.
Head Play isn't really an option, as he should be in the wings, waiting for a ball to use the Break In, not waiting in front on the box for a cross.
Slide Tackle is not a bad option, but it's just like the CF, only one slot and get Slide Tackle on it? He does get some more mobility, on the wings at least, so it's marginally more useful here than on the CF.
Nutmeg is only relevant if you have a full back blocking your path to the corner tile to use the break in. Not really useful.