Who knows. Is it better to concentrate on just a few skills or train all the white skills equally? Some people say skills don't matter at all.
Fitness and Speed are popular skills. There are three good drills for them: Long Run, Sprint, Slalom Dribble. Don't mix them together though, it would be pointless, choose only one. Try to read tooltips and think, it is part of the fun, and no one knows the right answer anyway. Just try to avoid training grey skills.
Check this too: https://forum.topeleven.com/tutorial...g-planner.html