re: Which is the best subgenre, in terms of overall quality, of Horror?

Out of what you listed, I don't see how you classify Seven, Terminator and 2001 as horror movies. But to somewhat answer your question, zombie, demon and found footage horror movies are the worst especially the found footage. The Blair Witch Project was barely passable as a decent movie, but these Paranormal Activity movies are so bad it's funny. Well, I actually only saw the first, but I don't think anyone is going to tell me they got better as they went.