Seeing as TG Cid has bee mentioned a countless of times, I'll go with this weird choice.
this dude isn't really overpowered per se, in fact his stats are pretty goddamn awful. but he has one attack that's amazing for farming exp crystals (the best in the game actually), once you learn how to time it well with the rest of the party attacks, behold exp crystals heaven. he effectively breaks the game, simply because in each battle you gain a lot of exp. thanks to the exp crystals even battling normally and not grinding, with him in your party you can get overlevelled really quick.
nah but seriously the really overpowered dude in the game is
this dude isn't really overpowered per se, in fact his stats are pretty goddamn awful. but he has one attack that's amazing for farming exp crystals (the best in the game actually), once you learn how to time it well with the rest of the party attacks, behold exp crystals heaven. he effectively breaks the game, simply because in each battle you gain a lot of exp. thanks to the exp crystals even battling normally and not grinding, with him in your party you can get overlevelled really quick.
nah but seriously the really overpowered dude in the game is

