Until the era of the Super Nintendo, final bosses in RPGs were nearly all the same. They were evil entities that loved being evil because they loved evil. Their motivations were pretty much “Mua ha ha ha!” and not much else. Not only that, they were generally heard about and not seen until the final battle.
Kefka turns all of those cliches on their head. We actually get to meet him very early on in the game. Not only that, he’s actually weaker than your party and doesn’t stand a chance against you in battle. You also don’t KNOW he’s the final bad guy when you meet him. There’s a sneaking suspicion, but Gestahl has already been established as the leader of the baddies, so there’s room for doubt. Finally, he has a freaking personality. He’s arrogant, cocky, impulsive, and has a wicked sense of humor. Even though he’s weaker than you, he has the self-assuredness to mock you at every turn.