Roman Polanski’s Repulsion was one that made theater audiences jump in its time. I knew people who took the mirrors off their doors after seeing it.
Jacob’s Ladder.
I win, case closed.
When I was a kid, I saw Vincent Price’s The House On Haunted Hill, and it scared the hair off me. I had bad dreams for a week afterwards.
I saw it again on TV probably 20 years later on some late night creature feature. I settled in with my popcorn, all ready to be scared again…only to find out it was one of his “comedy/horror” movies, and a fairly sucky one at that. I still wonder what was going on in my life when I was younger that it scared me so badly.
No movie has ever scared the hell out of me like Alien did. In a theater when it was brand new. I was 36 but that film turned me into a ten year old.
Edit: add me to the list of people who think Blair Witch was well done.
Does anyone remember a movie from probably late 60s early 70s that was several short stories and I want to say starred Vincent Price but I’m pretty sure that’s not correct. It was another one of those creature feature type things and the gimmick was when something scary was about to happen they’d sound an alarm and the screen would flash read. After the commercial break they’d show the message “when you see red, turn your head”. The only story I remember had some kind of bad guy who was being transported on a train to jail or something. He was handcuffed to some metal apparatus and the train got derailed and he had to hack off his hand in order to escape. I don’t remember if the rest of the story was about his disembodied hand going on a murderous rampage or what, but that scene stayed with me a long time. Some kind doper told me the name of it years ago but I have long since forgotten the name. I’d love to watch it again to see how cheesy it actually is.
I’ll also second Jacob’s Ladder, though I found it more creepy than scary. It’s the first instance that I can recall of that fast motion thing that’s since become popular, when you see the patients at the hospital with their jerky head movements. Also the scene at the party when Time Robbins hallucinates his girlfriend turning into a demon. Brrrrrrrr.
I agree with both of these. Like Play Misty For Me, a few more that freaked me out because I saw them at a young age:
I thought that version was very well done - and I could really see how that happening to someone could seriously cause mental issues.
Truth can be worse than fiction [mainly because it could happen to you or to a loved one.]
I have not really been frightened by most movies, but Misery and how real it could have been bothered me enough that I had to actually stop watching it. The secret nut cases that are all around us [sort of, anybody could turn out to be the psycho next door, like Ed Gein] make a movie like that very uncomfortable. Having been an abuse victim makes me understand how easy it is to be injured or tormented in private. In the same vein, I would not be able to watch Gerald’s Game or Rose Madder if they were made into films.
It wasn’t a theater movie, just one third of a made-for-TV flick called Trilogy of Terror, with Karen Black being terrorized by a Zulu doll. Might seem a little tame now, but for its time it was pretty terrifying. The doll was like a little Terminator, if the Terminator was a ball of razor-sharp teeth.
Even now, the still images make me feel a little sick. No way I’m watching that video.
Thirding The Ring for an entire movie.
I do admit that the scene in Blair Witch Project where the high-pitched shrieking voices start and the sides of the tent start to shake is the worst for me in terms of long-term effect. We camp a lot, and inevitably when I wake up in the middle of the night, my mind snaps to that scene (where I SWEAR if you look closely enough you can see the outline of little tiny hands beating on the sides of the tent) and I have a very hard time going back to sleep.
In general, that movie didn’t scare me, but there was that ONE SCENE that I’m sure you remember. When the female ghost comes gliding across the screen, claws outstretched and face twisted into a grimace. I nearly jumped out the window!
Ha! So did I! How strange. I was actually home alone, after dark, in the basement when I saw the movie. I have scars. 
Hush Hush Sweet Charlotte…viewed as a little kid. And a great cast too.
Not so much the movie, but my age. I was 6 when I went with my older brother and his friend to see “The Beast from 20,000 Fathoms,” which I found terrifying. At one point I was so scared, I had to pee, but my brother wouldn’t get off his lazy ass to take me to the men’s room. Ultimately I peed in my pants.
The Ring is certainly up there for me. I saw it as a teenager in the theater with my father who was wholely unimpressed. But it was honestly just a super fantastic mystery movie that happened to have maybe 2 or 3 truly extremely frightening scenes (flashing to that girl in the closet, as already mentioned, has to be one of the most effectively scary and gruesome things any movie has ever done; I still close my eyes ahead of time when I rewatch The Ring when I know that part is coming because I can’t stand seeing that face and I’ve effectively blocked it out of my memory now).
As a kid I saw Evil Dead and that scared the bejeezus out of me.
ETA: this doesn’t quite fit the spirit of the OP, but the most scared I’ve ever been in my life from something artificial (ie, I wasn’t in any actual danger), was one of those dumb internet screamer things. It had very low playing middle-eastern music, and you had to turn your volume WAY up to hear it well. Then it was posting the lyrics along in english so you could read them phonetically but not understand them of course. Then BAM evil demon face and horrible scream. I nearly had a heart attack. I think it took 20 minutes for my heart beat to come back down.
Threads. Watched it’s original BBC airing when i was 13. Didn’t sleep that night, had nightmares for a week.
Oh for those unfamiliar, it’s basically the British version of “The Day After”, but its bleakness makes that film look like the Wizard of Oz.
I’ll fourth The Shining. It scared me when I was little, and it still makes me feel uneasy all day long if I watch it now. I know why it scares me, because Nicholson’s doing the sort of thing people actually do, with Kubrick turning the visuals up to 11.
Alien also scared the bejeezus out of me when I was young. These days I’m just not that scared of alien monsters. If I were in a spaceship these days, I’d be far more frightened of pulsars and the vacuum of space.
I’ve been watching horror movies since I saw Nightmare on Elm Street when I was 5, back n 86. I never was scaredby a movie until Blair Witch. That thing had me terrified to go into the woods for months. It didn’t help that I bought into the whole found footage bit as well.
The first couple of paranormal activity movies creeped me out as well.
I never watched any of the popular horror movies that all my friends watched growing up. I’ve never seen Friday the 13th or Nightmare on Elmstreet or Halloween. But somewhere in high school I got into psychological thrillers (not all ‘scary’ movies, some just intense dramas). My nomination will be for Silence of the Lambs. It’s the only movie that ever made me say “Hmmm, maybe I’ll just sleep on the couch so I don’t have to get up and move around in the house”.
I was also a kid when that came out, and I was convinced- without ever seeing it- that it was the scariest movie of all time. I was was also convinced (also without seeing it) that Rodan must be the scariest monster movie of all time. Must’ve been effective advertising.