I don't really 'hate' any anime, to be perfectly honest. The way I see it I either can, atau 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 favorit anime would probably be Vampire Knight, atau Lucky Star.
My least favorit character is probably Asuka Langley from Neon Genesis Evangelion, atau maybe Ichigo from Bleach - he's seriously annoying, that guy.
That's easy and in my case, it's BOTH an anime character and an anime series: Milk-Chan from The Super Milk-Chan Show. Pretty much hate them with a passion