For me, scary movies have to be divided into movies i saw as a kid and then movies as adult.
KID: Exorcist, Aliens, The Shining, Ammityville Horror
ADULT: Blair Witch Project (I don't know why, it just freaked the pee out of me)
Part of the The Ring were deliciously scary too, although i wouldn't say it was overall horrifying.
It's hard to make a quality horror film that really gets under your skin. Doesn't happen too often, that's why lists like these often have a lot of the same entries. The REALLY good scary flicks are few and far between. The rest is just blood-splattered teen schlock.