I believe Terra is the strongest in terms of sheer power. Not really a fan of VI, and haven't player it in ages, but she killed some ridiculous number of opponents alone, and yadda yadda yadda.
My personal preference would be Auron. He died, and lived to tell the tale. That's badass. And he can break damage limit LUL GF OTHER GAMES. No not really, that's a horrible reason.
Cloud and Squall I'd put about on par with Auron (**** Tidus <.<). But between the two of them, I'd put money on Squall. If Cloud wasn't busy angsting or emoing every 11 seconds, he may have been a more sucessful hero. But as is, he's kinda lame. He never even made it into Soldier while Squall was one of the top in his class in SeeD, he stood by helplessly and watched his girlfriend impaled while Squall went out into deep space and saved his, Omnislash gets 15 hits to Lionheart's 20.
It's harder to compare heroes than villains, since the story purposely doen't show them being particularly overpowered in most cases, since that'd kinda defeat the point of a game in which your goal is to beat a superior nemesis.
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