Cutie Mark Crusaders don't have a leader, they take turns in it. In the episodes you see one of them leading...? leadering...? dunno. Like "Twilight Time" being Sweetie Belle's turn and "Flight to the Finish" Scootaloo's. I would also say that she isn't mature enough. I see her a little incompetent but that's okay because she's a child, they all are.
If I would choose a one that would be the best for the leader it will be Sweetie Belle. She's the most mature, she handles the group pretty well (Twilight Time) and she is like the quiet voice of reason (One Bad Apple when she was the one to tell Applejack and Flight to The Finish where she was the one to stop the fight between her friends). But overall it's just my opinion. ^^
And I would love to hear how you think about it too. ^^