@Shademp: Ying and Yang is a brilliant answer.
Took me long and hard to think of an enemy I hated more than any other and I'm afraid I could not make a decision, but here are my top five hated:
Golden touch frogs rub me up the wrong way. If one more of those fuckers uses frog song on me after I've selected a magic spell, I swear to God...
Nobody of a low exp level or at least one party member wit a Ribbon messes with a Marlboro. It's gaming suicide.
Dorky faces are bastards
Can't remember their name, but those red leaves that you fight in the Ancient Forest. Man, they do my tits right in.
And finally, the one and only Christopher, simply because he is arguably the most randomly named character in FF history. And he's a half fawn - half centaur fuck up