Favourite? Too many to pick - Cronenberg, Argento, Romero...Carpenter, Fulci and Hooper when on form, Terence Fisher, Soavi, De Palma, Mario Bava...and on
As for least favourite, if I can include him as a "horror" director (he isn't quite, but let's stretch the definition for sake of a rant, eh?
) then M Night Shymalan. I have loathed all of his films that I have seen. Indeed, "Signs" is my all time cinematic turkey. That this man is constantly given the opportunity to to make films (and high budget films at that!) leaves me reeling with confusion. He is the Michael Bay of fantasy cinema. No, he's far worse than that.