Mercer Frey. If the voice actor who'd done him had only done him, then it would be okay. But the guy's voice shows up in so many different characters and they all sound exactly the same. The guy has one voice and that's it. For me, it's the most immersion breaking experience in the game because no matter who the character is supposed to be, they all sound like Mercer Frey.
Belethor, Viarmo... gods, try to listen to his King Olaf's Verse! Basically he does every third named male NPC in the game. However, I can't give him top honors for annoyance, even if he's ubiquitous. Number one awful voice is the very, very bad imitation of Max von Sydow. Esbern required a lot of extra dialog, and either they couldn't afford von Sydow or lost his address, so they went with someone who sounds cross between Kermit the frog and Yoda. Gah!