I think we've had this topic many times and my core 4 never changes:

Cloud
Yuna (FFX version)
Orlandeau
Terra

With the right materia setups, Cloud is one of the most overpowered characters in any game ever. KotR and Omnislash in particular can one-shot most anything.

The only character that may be more powerful is Yuna. The Sphere Grid creates some incredibly beefed up characters and ultimate weapons can break the damage limit. Combine that with her overpowered Aeons and it would really take an army to take her down.

Orlandeau is another character that breaks his game, especially with Excalibur's auto-haste. He pretty much can one-man most battles in FFT.

Terra is more of a personal pick since I like her character. She's not more powerful in her game compared to the other characters (any of them can do the devastating genji glove-offering or gem box-economizer combos) but her esper form does give her potential to have god-like abilities.