Nightmare on Elm street !
Hitchcock is the man that knows how to fill me with fear.
Psycho, to me is an unnerving film. Not relying on SFX, it is shown in black and white, adding to it's haunting element.
The Birds is equally difficult to watch.
The man is/was a gifted storyteller, knowing fully how to deliver an uncomfortable, intriguing film.
Also Salems Lot, the one with David Soul. That snaggle-toothed vampire was just about the most hideous creature I'd seen in my young life ...
In terms of recent(ish) stuff, I went to the cinema to see The Ring when it first came out, not really having a clue what it was about. There's a grotesque flashback moment when they discover a dead girl in her bedroom, her face all twisted and her head sort of lolls forward, that was 'orrible. And my blood went cold when that thing climbed out through the TV.
When I was a kid, Alien PROPER shit me up. Not so much the famous chest-bursting scene, more the innate terror and tension as they walked round those dark corridors.