i say Kuja: teamed up with the queen of the second strongest nation in the world and used her army and his magic to defeat the other two nations.(Lindblum and Buremica/Cleyra)Then killed her and wiped out many of the alexandrian army along with their fleet of warships.

Then attacked Alexandria using summon magic.
Destroys Terra and Annihilates your team, by now he has also escaped from your team many times. Finally faces your team again kills them but then saves them. What a character