Dullahan from Golden Sun. Highest HP in the games (which regenerates a ridiculous 200 per turn), huge amount of PP (which regenerates a full third per turn), the best stats in the game, save luck (the person who beats him has 1 point more) and agility (this time he's beaten by one monster and a palette swap family of another one), and he gets 3 turns per round which he will use to destroy your party. Oh, and his attacks include stuff like the second most powerful summon in the game, attacking both you and your stat boosting and special ability granting Djinn, and hitting you with the most insane status ailments in the game.
Most people I know report only having the first party last 2 turns before he kills them off. Hell, a lot of the suggestions for beating him are to have max level before facing him and even then to pray for the best.
Otherwise, killing the final boss of MMZero 3 is bragging rights (3 rounds), Uncharted 2's final baddie, the tank from Jak II (satisfaction, not accomplishment), Night Terror from Soul Calibur 3, any of BlazBlue's unlimited baddies, the bonus boss in Tales of the Abyss, and quite a few others would rank up there for me.