I loathe two dimensional characters - the kind that are all evil or are all good and with nothing to either show they have some sort of conscience in the case of the baddies or flaws in the case of the goodies. I always preferred Jo and Amy in Little Women to Beth. The latter was a bit too saintly for my taste while her sisters, including Meg, have enough flaws to make them rounded characters, Jo especially. I don't know if she was Louisa May Alcott's favourite creation but it would never surprise me if she was.
I loathe slimy characters (as I do in real life!) though if an author has set them up well and you can see why they're slimy/what they're hoping to get out of being slimy, I can then go on to hope that said writer has set them up for (a) redemption or (b) a horribly sticky ending! I do like justice to "out" in fiction given how often it doesn't in reality. If there was one particular thing Roald Dahl got right (and there were loads), it was in dishing out deserving horrible endings to those of his characters who plainly had it coming!