WoW polls: the toughest Heroic instance

Blizzard has posted a poll on the forums asking what players think is the toughest Heroic dungeon in the game. Unfortunately, they don't say much else besides that, so guessing the reason why will be up to us and our commenters. They want to make sure Ulduar is tougher than Heroics? They want to look for particularly tough mechanics for future instances? They just enjoy seeing players have a