For me, it's Earthmender Norsala. I can't even tell you guys how much. It's specifically the way her voice would just boom out over all of Deepholm - I don't know if that was a bug or intentional, but I hated it. Like fire. I wanted Glop to explode in a shower of fragments just so she'd stop yelling at him.
There are other NPC's I don't like - Sindragosa was by far my least favorite boss in Icecrown Citadel (and I'm pretty sure she's being voiced by the same actor - perhaps I just am not fond of shrill screaming voices, come to think of it) and I generally have decided I dislike John J. Keeshan for being a bad movie reference. Oddly enough, I don't mind the rest of his squad, just him. Maybe because (spoilers for a Cataclysm leveling questline) they all die.
So how about you? What NPC do you absolutely loathe?