It'd have to be Delores Jane Umbridge, from JK Rowling's Harry Potter and the Order of the Phoenix. Particularly, in the film, her sweet but sinister tone drives me insane and the entire "You broke a rule!" aura about her actually makes me tense up inside and grit my teeth. I can think of no other character who evokes such a hatred in me other than Umbridge.
Yourself?
Yourself?