Gilgamesh.
I was fighting him in FFV Advance, Exdeath's tower earlier (the 'Excalipoor' battle) and it was the most fun I've ever had in a FF battle. It was pretty great how he spoke throughout the battle, used a wide variety of moves rather than a limited moveset, lasted so damn long, then revealed a second form ("For Gilgamesh... It's morphin' time!") and continued to be awesome. Then Exodus/Exdeath ruins everything by throwing him into another dimension. Dumb. I was thinking out loud "Um... We were in the middle of a fight, and he was doing pretty well, why banish him?" Oh well.
Anyway, Gilgamesh is awesome as hell.