A quote by me on another forum:

"Girl [Mikoto]: But Garland is dead. Terra will soon be destroyed.

This was Zidane talking to Mikoto shortly before they boarded the Invincible and escaped Terra. She makes another comment on Terra later on the beginning of Disc 4.

Mikoto: You saw Kuja's power. He destroyed a world by himself...

Now this is where semantics come into play. What does it mean to destroy? What is a world? What is the definition of “is?” People accuse, and they are kind of right, those lines of being too vague to attest to the fete of busting Terra. Yet the idea Kuja could blow up a planet did not materialize from nowhere. The fact is that there is not 100% conclusive evidence that he did or did not wipe out Terra. But, I feel just the two quotes I provided completely disprove the absurd notion that I’ve heard here and on other places that Kuja only destroyed Bran Bai. Some say he just tore down the city and looking through both those quotes, I don’t see her saying anywhere that Kuja destroyed a city by himself or that Bran Bai will soon be gone. But, nevertheless, I am a supporter of the planet-buster Kuja school. And partially out of faith and partially out of reasonable conjecture. Such as drawn from this line.

Amarant: Do you really think [Kuja] perished with Terra?

Now, examine that quote. If Kuja did nothing but strafe the world with magic, how would he perish along with it? I deduce, and I feel I’m definitely not going out on a limb here, that Amarant is saying Kuja did indeed blow up or cause some sort of cataclysm on Terra. How else would he “perish” along with it? When I hear that line, I have an image of a planet exploding and Kuja caught up in it. Does that seem too farfetched or reaching? I don’t think so. I do pride myself on having an open-mind. I can admit to being wrong and can look at the same line and see multiple meanings. But I just don’t see how that quote could say anything other than what it’s saying."

Anyway, Sin is out of the running mainly because it could be beaten by kuja or Kefka and it's not a villain... It's only down to Kuja, Kefka or Neo EX-Death.