Tidus is already dead so hes excluded....
Given that Advent Children is canon; Cloud (FF7). Not only did he take a "swim" in what would be Midgar's version of radiation, he can fly for short stints and beat summons with his sword, and apparently it doesn't even need to be the buster sword. Or if we're considering Kingdom Hearts as some sort of canon/age of apocalypse universe; still Cloud, not only did he beat Sephiroth in an alternate dimension, the darkness (that which everything runs away from because only keyblades can beat it (them) was unable to make him submit. In their own universe Squeenix kinda makes Cloud Superman.
For the sake of discussion; Vincent would easily be number 1, he really can't die, he can transform if he's knocked out, nothing beat him...ever, not even the emo kids in AC, in fact, he saved Cloud in the forest. It's between those two.
Honorable mentions; Zidane (being an angel of death, it needs no explanation). Terra (she's pretty much a goddess), Zack (He did beat Sephiroth on the 1v1 but he lost to bullets...Cloud dodges bullets at one point.