Any. Freaking. Archer. No matter what race/creature. Go against them in a group and you're gonna be dead really quick. In packs where they are mixed in, I always take them out first. At least you have some kind of chance against basic melee mobs.
Draugr DeathLord Archers, that ones that can actually make you go ragdoll.
And Master Volikhar Vampires. I'm on PS3 so I don't have access to DLC enemies.
Well the masters can be found in Haemer's Shame, without the DLC.