Shut up and tell me the best drill!
Here's a list of all key skills and all "clean" drills (that don't train grey skills) for every single position. And also leftovers -- key skills that you can't train to this position without training at least one grey skill. If your player has several positions, he also has key skills and clean drills from all of them, plus probably a couple clean drills more. Use ITP if you wish to know all the clean drills for any combination of positions, and stuff.
Goalkeeper Training drill is good for everyone, so I don't include it.
Defenders
GK
Clean drills: Set-piece Delivery, Wing Play, 1-on-1 Finishing, Defending Crosses, Hold the Line
Leftovers: Fit
DC
Key skills: Tack, Mark, Pos, Head, Brav, Fit, Str, Aggr
Clean drills: Press the Play, Hold the Line, Warm-Up, Gym
Leftovers: -
DL/DR
Key skills: Tack, Mark, Pos, Brav, Cros, Fit, Aggr, Speed
Clean drills: Press the Play, Hold the Line, Carioca, Long Run, Hurdle Jumps
Leftovers: Cros
DMC
Key skills: Tack, Mark, Pos, Head, Brav, Pass, Fit, Str, Aggr, Creat
Clean drills: Press the Play, Piggy, Use Your Head, Video Analysis, Hold the Line, Warm-Up, Gym
Leftovers: -
Attackers
MC
Key skills: Tack, Mark, Pos, Brav, Pass, Drib, Shoot, Fit, Speed, Creat
Clean drills: PGS!, Slalom Dribble, Video Analysis, Hold the Line, Sprint, Long Run
Leftovers: Tack
ML/MR
Key skills: Pos, Pass, Drib, Cros, Fit, Speed, Creat
Clean drills: Slalom Dribble, Sprint, Long Run
Leftovers: Pos, Cros, Creat
AMC
Key skills: Head, Pass, Drib, Shoot, Fin, Fit, Speed, Creat
Clean drills: PGS!, Skill Drill, Slalom Dribble, Sprint, Long Run
Leftovers: Fin
AML/AMR
Key skills: Pass, Drib, Cros, Shoot, Fin, Fit, Speed, Creat
Clean drills: PGS!, Fast C-Attacks, Slalom Dribble, Sprint, Long Run
Leftovers: -
ST
Key skills: Pos, Head, Pass, Drib, Shoot, Fin, Str, Speed, Creat
Clean drills: PGS!, Skill Drill, Shooting Technique, Use Your Head
Leftovers: -