I don't really 'hate' any anime, to be perfectly honest. The way I see it I either can, ou can't, get into an anime, which is to say that it's not something I enjoy - but, hate is a very strong word and I won't use it for anything to do with anime. Besides, if I don't like an anime, I won't watch it.
My least favori animé would probably be Vampire Knight, ou Lucky Star.
My least favori character is probably Asuka Langley from Neon Genesis Evangelion, ou maybe Ichigo from Bleach - he's seriously annoying, that guy.
That's easy and in my case, it's BOTH an animé character and an animé series: Milk-Chan from The Super Milk-Chan Show. Pretty much hate them with a passion