Ancano always reminds me of Lucius Malfoy. And I hate both.
Rolff Stone-Fist would have to be my #1 hated NPC. He's the biggest racist in the game (after Ulfric), and for some unfathomable reason he's also essential. AND if you beat him up, he considers you a freaking friend!
One time I had to rescue him from Morvunskar (thanks, Vilkas)... and he acted like an utter idiot. He'd take a few steps, then stop for twice as many seconds. The flame trap outside would've killed him if he could die (I admit, I deliberately triggered the trap
). By that time I was so frustrated and angry that I gave in to the temptation to use Ice Form on him. Hardly appropriate, especially since it was a Companions quest - but oh so satisfying.
Number two is definitely Brynjolf. Whether or not (nearly always the latter) I join the TG, he always gets on my nerves at some point. Maybe one day I'll actually finish the questline (as in get the final achievement) and see whether he'll still be his usual self.
Like so many others, I don't like Delphine. And would dislike her even without the whole ultimatum thing.
Jaree-Ra is another extremely unpleasant individual. Especially since he only loses his essential status if you do his stupid quest.
Most kids are annoying to insufferable.
Maven doesn't bother or annoy me, and neither do Heimskr (seriously, that guy is so devoted to Talos that he'll preach throughout the entire battle for Whiterun, even if you side with the Stormcloaks) or Siddgeir (granted, he's not that good a Jarl, but I still like him).