Having voted on the poll but not having read others' responses yet:
I chose The Imprisoned. It was kind of a tie between him and Tentalus. Both were frustrating multi-stage bosses and both looked fragging ridiculous. Tentalus looked like some kind of child's bath toy to me... the Imprisoned...
as I say on the Skyward Sword Meme that I have on my Deviant Art page, The Imprisoned: Me Am Doom-Muppet, Rwar!
It looks like evil Elmo with a manticore's mouth! Also, as said on my DA meme-doing, part of fighting it reminded me of "Shadow of the Colossus" which had me thinking "Why aren't I turning this off and playing that? It's a better game than (this portion) of the game I'm playing!" (Don't get me wrong, Skyward Sword is my favorite game right now, as an overall game, but that part of the boss-fighting was obnoxious and more obnoxious for feeling like it had ripped off another game that I think is awesome and does the whole "climb on something and whack the weak point" in a much better way). In other words, Skyward Sword is awesome as a whole game, but I'd like to see one of the Colossi stomp the Imprisoned into the ground.