City of the Living Dead for me, with Zombi and The Beyond close behind. City just oozes with atmosphere, and has some great gory deaths. Zombi and The Beyond are really good as well, with great sets and gore, but they seem to lack that creepy atmosphere that is present in City. House By the Cemetery is decent, but I don't think it's as good as the other three. Bob is awesome though! As for Zombie 3, I haven't watched it yet, even though I have owned it for a few years now. Just haven't been in the mood to watch it considering its terrible reputation.