I'd have to pick Terra; she was just too awesome. Her strength in Vigor and magic was something new to female characters. The old ones just made the girls really weak and they missed a lot and they sometimes ave them high magic skills. But Terra has both and when I played through the each release of the game, my Terra was always the strongest on both arts. She was a great character that had been developed perfectly with deep depth 
(Second would be Edgar because of his mad skills with the Tools and him being a lancer/swordsman and than third and fourth would be Celes and Locke)

(Second would be Edgar because of his mad skills with the Tools and him being a lancer/swordsman and than third and fourth would be Celes and Locke)