Zant (hence the avatar), up until you fight him (which his wimpy easiness just ruined him) he was just so cool and creepy at the same time, his theme song sent chills down my spine and the cool/weirdness of him just made him awesome.
Sorry, but I've got to show some love to <3Sheik<3. She's the greatest character ever, and I have a thousand reasons why. BUT if i can't chose her due to the fact that she is one of Princess Zelda's many alter egos- then i'd have to say its a toss up between Skull Kid, and Midna. I know most people despise Midna, but you have to admit- she's the least annoying sidekick, and she has the best back story in my opinion. (ALSO SHE IS GORGEOUS!)
The Twinrova battle from Ocarina of Time was very entertaining to say the least, then again Demise from Skyward Sword was also one of those keep you on your toes battles.