ExDeath strikes me as the most powerful character, given his ability to suck whatever he wants into the void.

Major Lightning Returns spoilerThen there is Lightning, who effectively is turned into a Goddess by God, and then she beats up God. So... yeah... more powerful than God is pretty much as powerful as you can get, surely?

And finally, there is every one of the playable characters in FFXI and FFXIV. They are effectively immortal, every one of them. They have an eternal ability to revive themselves retaining all knowledge and experience gained from battles they are defeated in.