Spoilers Ahead

Definetly Albedo!

Although he had a reason for being th antagonist, he is probably the best one ever. The one scene where he shows MOMO his invincibilty, by ripping off his own arm, and his head, and making them reapear.

The lines in the fight are awesome as well. For instance, when he uses Ruby Fist: "C'mon, C'mon, C'mon, C'mon, C'mon! Now die!"

Or when he uses Pheonix Wing: "Burn in the depths of hell!"

Kefka comes in a close second, followed by Sephiroth.