I would have to say that two of the scariest films I've ever seen are:
the original Ju On and
GhostWatchJu On, while not being scary to the point of making you jump out of your seat - or as Bruce Campbell puts it "Punching the guy sitting next to you in the arm," - it is very creepy, and some of the deaths/disappearances give the feeling that you can be taken from (literally) anywhere. That, and the kid is real creepy. I can't tell you the amount of times I've walked past a mirror and expected that kid to be there.
GhostWatch was a typical piece of British film making. It started out really light hearted and fun, then went for a drive through creepy, past scary and into down right crap-your-self realism. It was presented as a "live" show when it was first aired (note: it's only ever been aired once), and the public took it as that. It's filmed like a documentary, with several scenes filmed 2 or 3 times to add an air of suspicion in the viewers mind. Plus, the antagonist is only ever seen for 10 to 15 frames at a time.
Zombies movies
CAN be good, more often then not they're just an excuse to use violence in a movie. That being said, they do seem to be the easiest of the genre to make.
@tak197: Whilst I've only ever seen Psycho, I would have to agree with you. 15 gazillion people in the world cannot be wrong about the genius that is Hitchcock. Plus, my favourite place to eat is named after him.