@Gilthanes: If a character fights Superman, and has access to Kryponite, it's not an accurate indication of his or her strength. Same goes for Sin and the Hymn. And where did you get the idea that X's heroes were more inexperienced than most? II, III, V, VIII and X-2 had no members older than 19 (considering Galuf was a temporary character); and of the other games, X has one of the highest ratio of characters > 20 years to characters < 20 years, only beaten by VII and possibly IV and VI (I can't find a reliable source for the ages of the latter games.

@Elpizo: You claim that Cloud of Darkness, Zeromus are invulnerable, which I'm not disputing, but this thread isn't called "What FF Villains couldn't be beaten by Sin." Sin is just as invincible, so it would end in a stalemate, not in Sin's death. You also seem to be forgetting about Giga Gravitron; the fact that it results in a Game Over even if you have an Aeon out means it's not just a flashy attack like Particle Beam, Big Bang, or Supernova. Plus the only reason you could even have time to defeat him before he uses it is due to Jecht's influence and the Hymn.

@ The Crystal: Evidence?

@ Renmiri: I didn't even think of that point about Jecht holding off Sin. Thanks for that.

@Serapy: Are you thinking of Giga Gravitron? It's the move he uses if his OD bar fills up, and it pretty much the same kind of attack he uses in cutscenes.