That merchant in Broken Valley who sold items that appeared to be magical in the inventory screen but were fakes. Very intelligent of Larian. I liked that but hated the NPC, I wish Divinity II allowed us to kill everyone we came across because he would have been one of them.
Reading his mind allows you to figure out which item isn't a fake.
Tom, Dick and Harry. < I laughed when I first heard their names but I really hate them. "Argh! Lord..Whaaaaaaaaaa! I'm injured..HEAL ME! WHAAAAAAAA!" These are my personal scouts?
Well, that's your own fault for not investing in some decent equipment for them. Though I did find it annoying that I couldn't just shell out for the best stuff right away and was instead forced to buy weak and medium stuff first, driving up the costs.
Anyways... my most disliked NPC would probably the patchwork doll Sassan. The lisp, the bad attitude, the stitches... everything about that necromantic abomination just rubs me the wrong way.