Adaon. He robbed you in your sleep. Never forget.
I miss the version of the game that let you push Companions into traps. I could spend hours pushing Adaon into all kinds of different traps. Fire traps, bear traps, spike traps. Now, I have to tell him to Stand There while I trip a gas trap. It's just not the same. It lacks that personal touch.
But, in fairness, let's discuss.
Therod Nom is merely grumpy from a rough boat ride. He trades you one of the best early-to-midgame amours for a mere minotaur horn. And you want to repay that generosity with public shaming? Such nonsense.
Andrys Storme is a good cotender for top hated list. He's a snobby, evil, vampire suck-up. But he's upfront with his distaste. And you run into him at his place, so shame on you for barging in. Plus, you get to teach him some manners, and bring down his entire house. So, evens out.
The Magister is another top tier contender for most despised. He drugs you when you first meets you, and spends all kinds of time trying to kill you. He'll even trade all the world for his own immortality. Rather self-centered prick. But, he listens to reason. Show him the error of his ways and he'll repent. Even gives you a nice make-up gift (do check it for poison).
The Conjuring Librarian for the Golden Hand rounds out the top tier list. Demands loyalty in exchange for access to a library? The library lacks a decent selection on books, too. Less of a Librarian and more of a pompous doorman with an inflated sense of self. Don't feel bad about destroying him and his pet toys. Just reminder to speak softly when bashing his goons. It's a library, not a battlefield.
But, Adaon. Oh, that little creep.