Nurse Ratched form One Flew Over The Cuckoo's Nest. What a horrible character! Louise Fletcher in the film of the novel is impressive in her hatability also.
Good one, in terms of the movie. I haven't read the book because I unfortunately came across the movie first and it put me off. How would you say they relate? I can't help but think that so much of it must be internal that the book would be better than the movie, but I don't know.
(To tie it into SF, Fletcher also played Kai Winn on DS9 where she played a more complex but usually equally hate-able character.)
Oh, topic. Well, there's two different things going on here. We can hate a character by intention in which case it's an accomplishment on the part of the author or we can hate a character in a way the author might not have intended, in which case it's likely a failure.
The most recent extremely despicable characters I've come across are
Bella Lind and the even worse
Svetlana Barseghian in Alastair Reynolds' otherwise pretty darned good
Pushing Ice. They basically ruin the book. Both are basically incompetent and inconsistent and Bella is a masochist and Svetlana is a sadist. Bella's contemptible but Svetlana is gratuitously evil. Yet Bella is supposed to be a sort of "hero" and Svetlana is supposed to be forgivable.
Another epic fail (in my tiny, tiny minority opinion, but with some good company) is
Ender Wiggin from Orson Scott Card's
Ender's Game which isn't (IMO) a particularly good book anyway. Norman Spinrad brilliantly analyzes some of it (but his essay doesn't seem to be online) and
John Kessel covers most of the rest of it.
As far as characters who are intentionally and successfully unlikeable, I know there are some but I don't usually care for that sort of thing and can't think of good examples, unless maybe Dostoevsky's Raskolnikovs and Underground Men and such.