Any humanoid (which in Skyrim, seems to be any enemy with even the slightest leaning toward magic) that can cast a pesky frost spell. As my char is a dual wielding type, I rely mostly on getting in close to deal the most damage and getting hit by one of these spells that seems to slow me, appropriately, to glacial speeds is most infuriating.
Also, vampires, especially in groups. I recently stumbled upon a master vampire and his friends in a cave somewhere. Took many attempts of trial and error before finally coming up with a strategy that worked. Though calling "turn into a werewolf then charge into the middle of the group" a strategy may have been a bit rich.
In the end, equipping my two ebony maces and firing up the Elemental Fury shout made short work of that lot.
I don't really have any trouble with beasts...even the snowy variety, I can usually drop them fairly quickly now.
Hagravens, though I haven't met one in a long time. The second time I crossed one though, I threw myself off a sheer drop in my haste to get away from the deformed birdwoman. The fall probably still did less damage to me than the hagraven would have.