Star Magician, simply because it really got balls to the wall there.
He was the most original of all the bosses in all 3 games to date, his use of the balls to destroy my party made me rage yet got my senses up, the concept was pretty good too, he also was guarding my favorite summon, Azul.
When he used the guard balls I always got mad lol, when he used the anger balls I said "meh", Thunder balls was trouble but not terrible, Refresh Balls made me mad since he always heals himself >.<
The new Death Balls was quite evil of Camelot, but not as much as the Ghoul Balls since they ate my Djinn till I destroyed the balls.