Named characters only.

For me it would be Njada, she treats you like [censored], but then you become Leader of the companions and she suddenly becomes kind and tells you lots of words that land on your ears like warm water on your post-dentist teeth. (crappiest example ever)
No wonder why the wiki says she "also excels in Speechcraft"

Rolff Stone-Fist. I absolutely HATE this jerk off!!!!!

nazeem. even if you finish eveyr qquest available, save his life multiple times and he STILL seems to hate you.

For me, by far, it's Ancano. His arrogance and condescending attitude were the primary factors.

If you really hate Njada, take her as your follower for a few battles when you're around level 40 or above. She gets beaten so easily she spends most of her time groveling on the ground, which should satisfy any anger you have against her! I didn't like Ancano, but he was such a pantomime villain that I found it really hard to hate him. I was disappointed that he wasn't more fleshed out with redeeming factors (Elenwen is quite cool character, and not overly unpleasant, for example) and there were times when I felt that his character was more to teach us not to be bigoted against Altmer, and that somehow he would earn my respect or something. Sadly for him, he was just as two-dimensional as he first appeared, and died as just another maniac. Tullius was a bit of an idiot when it came to respecting Nordic culture and tradition, but his comments were usually more to do with scripted conversations with Rikke, and she always talks sense into him. Nevertheless, it's amazing how a man who is trying to be a figurehead of Empire authority in Skyrim can be so ignorant or disrespectful about Nords, but paired with Rikke his ignorance is made to be more palatable.

If there's anyone I really dislike, it'd be Delphine. Esbern really ticked me off with his "I found out who the leader of the Greybeards" lecture, but the way Delphine tried to gatecrash the peace negotiations was just arrogant. Esbern, at least, had the sense to focus on the urgency of the emergency at hand. Balgruuf's youngest kid who always snidely remarked about me being a dog here to lick his father's hand was also annoying, but since completing his quest he's now completely fine. Erikur also annoys me a lot with his condescension - but that's more poor writing than anything. I was so looking forward to seeing how he spoke to me after I became a Thane just like him, but he still talks down to me.

Jarl Siddgeir of Falkreath . This guy is an absolute poopface and I cried when I saw he was essential somehow