Playmaking
From Hattrick
This is the most important skill for all midfielders, especially the inner ones. It is also used by defensive forwards and offensive defenders. Offensive wingbacks use a tiny bit as well. As noted above, this skill is used in conjunction with stamina.
Almost every successful team needs good playmakers or else the team won't have a lot of ball possession, which will lead to fewer scoring opportunities. The only exception are teams who specialize in using the Counter Attack tactic.