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 tough time of it? Your guess is as good as ours.
Loken is notorious as a tough boss in Heroic mode, but his Halls of Lightning only makes second place, right above Ahn'kahet and its tough fights of Jedoga Shadowseeker and the insanity-inducing Herald Volaj. And it's worth noting that as of this writing, Drak'theron, The Nexus, Utgarde Keep and Violet Hold have a total of zero votes among them -- apparently everyone agrees they're easy as-is. But right there on top sits everyone's least favorite drake-riding instance -- The Oculus, with all of its drakey goodness.
Unfortunately, this may not help Blizzard -- I doubt many players would be really excited if they re-used the "drakes" mechanic in a lot of future dungeons, as the difficulty there lies in playing a completely different game rather than using the abilities of your character that you're used to. But It's an interesting poll anyway. What instance are you finding the toughest in Heroic mode?