The Draugr Motherfucking Death Lords, that's what. They always use that damned disarm shout. Every time. With Brearth I have to carry several weapons just in case I run into one.
That's not as bad as it sounds, though, since dragons don't have as many ways to make noise while in the air. If they're gliding they should be completely silent until they start flapping again or roar.
I have a mod that makes Dragon Priests harder... So yeah, them. Volsung specifically, though. The mod makes him weaker... but he casts illusion spells that make copies of himself and disorientate you so you can't tell where the real one is.