Most genre horror films try way too hard to be scary and as a consequence are anything but, you know by the music when they're going to make you jump, you know the irritating girl will get her tits out shortly before dying and the black character will be the last to die but will never be the survivor, you know it will take at least 3 attempts to kill the baddie etc
Psychological films or ones that play on genuine human traits are far scarier. "Jacob's Ladder" still sets my nerves on edge while most of the Lynch films particularly "Blue Velvet" & "Eraserhead" make me uncomfortable. The only one that as an adult has truly scared/repulsed me to the point where I turned it off was "Henry, Portrait of a Serial Killer", we had just dropped LSD at the time though which was probably not the wisest state to try and watch the film in, I watched it the next morning and still found it horribly real and very uncomfortable to watch.