I'd also have to say that knee-jerk shocks and bloody dismemberments (for the sake of gore) are, with VERY few exceptions....NOT scary.
What IS scary to me are the things that get under your skin and send a chill through your spine, make you question your place in the world, or just leave you with a vague sense of unease and being disturbed.
David Lynch's movie, not technically horror..... though his best, most surreal ones succeed in doing JUST that. Lost Highway, Mulholland Drive, Blue Velvet, Wild at Heart, Twin Peaks: Fire Walk With Me, Eraserhead..... all feature moments of sublime creepiness.
I also love the overwhelming sense of utter dread in the first ALIEN,
the nonstop unrelenting action of ALIENS, and the sense of isolation and futility in ALIEN 3. Hated ALIEN RESURRECTION.
Also, I thought there were several scenes in SIGNS that were unbearably scary and suspenseful....
I love the Romero Zombie movies, but more for their allegorical and apocalyptic view.
Now, John Carpenter...at his peak, his movies were the shit!
Halloween, in spite of its imitators, I think is the "perfect" horror movie, and will NEVER be equalled. All suspense, beautiful camera work, Carpenter's music score, likeable characters, and pretty much a complete lack of actual gore. WHen I first saw this, I slept with the lights on and a kitchen knife under my bed!
His remake of The Thing was great science fiction horror, and even in some of his lesser movies, he had a great command of mood and eerieness, even if they didn't add up. Prince of Darkness had many great creepout moments, for example.