For me I have never seen anything to beat the final scene of Carrie, where her hand comes out of the grave. When i first saw that at the cinema they turned the volume up at exactly the right moment and i jumped out of my skin and walked out weak at the knees. I have never seen any moment of horror since to top that.
It's a bit silly now, but the hills have eyes really made me cack when I first saw it - and Texas chain saw massacre...I've never picked a hitch hiker up since!
The Hellraisers get my vote - specifically Hellraiser 2!
Also, the Omen 2, I still hate lifts after the scene where the lift comes crashing down and the cord follows after and cuts the guy in two! - or the bit where the boy falls under the ice and they can't get him out!
There's a big difference between being revolted, being surprised or even shocked, and being truly scared. Personally, I don't think I've ever met a film that actually induced genuine fear in me. Maybe I'm not imaginative enough?
Definitely the Exorcist - me and some mates pretended we were 18 (were actually 15 at the time) as it was an X-film (as it was called in the 70's/early 80's). I spent the entire film slumped down in my seat peaking through my fingers - even now all these years later I still can't bring myself to watch it again. Oh and also Friday 13th (first one), I couldn't sleep for weeks after that, imagining someone was under my bed!
The only film I haven't seen out of those is Dawn of the Dead. I liked Saw and Hostel but didn't think they were scary but House of 1000 Corpses made me want to be sick at times. The sequel, Devils Rejects, wasn't that great or scary though...
I can't honestly say I've ever been scared by a film - I've been creeped out by one of two - but not genuinely scared. I think the one that creeped me out the most was probably the original version of Halloween.
I think that horror films that have been made since The Exorcist have desensitised us to horror somewhat. It was definitely extremely scary at the time when it first came out at the cinema, as was Jaws and Alien - there was massive media attention for each one, but we don't get that shock value any more because we've seen it all now!