Now that the final raid tier has been out for a long ass time, I think we may as well have a discussion on the boss throughout all three tiers that you hated the most. Could be because of mechanics, raid comp reasons or even just because they didn't drop your loot.
Personally, I hated Garalon the most. Fuck that boss, in an expansion full of fights that were fun as hell to Tank, Garalon was the one boss that I was forced to play a spec I really dislike (Ret) in order to progress. Bonus points to Lei Shi for just being a really fucking stressful fight to tank as a prot paladin.
I'd put a poll, but I don't think I'd be able to have that many options.