I'm pretty sure that once Yu Yevon was defeated at the end of the game, that the cycle of Sin was destroyed along with it (him?). Sin was only a vessel that protected Yu Yevon, like his armor or something. Once Yu Yevon was destroyed, there was no more need for Sin to "come back".
And as for who I think was the strongest? I'll go with what Rocky posted. Wakka ftw.
Bookmarks