As easy it would have been to mention the total toad that is Dolores Umbridge (no disrespect to the incredible Imelda Staunton, who depicted her perfectly in the films), I had to choose Professor Quirinus Quirrell for this one.
I think the problem with Quirrell is that we don’t get a backstory. Every other character has one. Even Umbridge, you have enough to loathe her with every fiber of your being. But for Quirrell, hes a bit of a weak professor with a stutter, then BAM he’s evil then BAM he’s dead. All we ever know about him is that he’s weak, and that Voldemort promised him glory. The major difference between Quirrell and Pettigrew is the lack of backstory.
So this is very much least favourite, rather than most despised, as I don’t think there’s enough on Quirrell to form a decent response. He’s just weak.