Yuna.

Like Necronopticous said, in the end of the power-up, everyone is the same, aside Overdrives. All magic, all skills, max stats.

But Yuna still has something that no one has. Summon. That makes her the best.

Yuna, Rikku and Lulu/Wakka = Pwn. Because Mix rocks. And I like Fury, it can deal some obscene amounts of damage.