I've yet to read all of her books but I'm starting to reread those that I do own.
I don't have a definite favourite yet but And Then There Were None, Nemesis, Why Didn't They Ask Evans, Halloween Party and Peril At End House are up there.
My least favourites are probably The Hollow and Elephants Can Remember. I can't even remember why I disliked them but I remember feeling very annoyed and disappointed when I read them. Hopefully I'll enjoy them more second time round.