No really, now seriously, the more close of what I think is the 4th. Manager have o be capable to test players and see the internal programming of players.
Tactics.... I defended always that all formations can win a treble wih the correct players...
Strong team is important but as I say every player have internal %'s of effectiveness, so some players have ahigher ratio of score, despite more or less stars every player have a min.-max. that can reach and is the manager who have to be capable to see that.

"Best leader" if you mean Key player, that is one of therse players that appear, contribute and that alone can win the 70% of a league title, then to find one of these players depend of the manager, and yeah, a key player is something that bless your team if you find a man like this.