I don't know about difficult... difficulty is such a weird concept in WoW to begin with. Most of the people in this thread equate the difficulty of REACHING a fight with the difficulty of WINNING the fight.
Instead, I'll say that the most FRUSTRATING fight in all of WoW's history is Auriaya, beyond a shadow of a doubt.
My vanilla guild ripped our way through AQ pretty quickly, but we wiped on C'thun EVERY single week until he got nerfed. We then wiped, day in, day out on Gluth (Back when Naxx was a 40 man) but never did wiping on a fight feel like slamming your dick into a brick wall covered in razorwire the way it does every single week to wipe on Auriaya 3-4 times, while everything else but Yogg and Vezax are on farm.
Too much chance RUINS fights.