For me, it’s either the 1944 version of “Gaslight”, starring Ingrid Bergman, or a late 1990s German movie called “Funny Games”, which is not funny at all.
I asked this question on another board, and one person nominated “All The President’s Men” which I found interesting, to say the least.
The Shining creeped me out.
*Children of the Corn. * Not that I think it’s particularly scary now, I was just too young when I saw it for the first time (under 10, exposed to it by the older brother of a friend). Scared the absolute living daylights out of me.
Didn’t help that at the time my somewhat remote house was surrounded on virtually all sides by corn fields.
Thought of this immediately upon seeing the thread title. It’s my opinion that The Shining may be the scariest movie of all time.
I can’t remember the name of the film. I was about fourteen. Don’t think it was supposed to be a frightening movie but in it was a shot at dusk of a graveyard and one grave had a life-sized angel stone. It’s face was crazed with cracks. That was the scariest thing I had seen in a film at that time.
I was certain for several years after that that if I opened my closet door at night that angel would be waiting for me in there.
As an adult? The first time I saw “Nightmare on Elm Street” I couldn’t take a bath for nearly a year. (And, yes, I do have a shower here.)
Given more thought, both the antiwar films, “All Quiet on the Western Front” and “Johnny Got his Gun” are way more terrifying than anything a scriptwriter can concoct totally from imagination.
Diary of a Madman (1963) with Vincent Price. I saw it when I was a teenager, so it made more of an impression that it would now. Price’s hammy acting seemed way too real to me. It’s based on a short story by Guy de Maupassant. Price’s character is possessed by an alien creature without a physical body that causes him to murder people he loves, and then frees him so that it can enjoy watching his reaction when he sees what he’s done. Oh, and without a body, it’s unkillable.
We probably need to differentiate between “Scariest Movie” (which is very hard to pin down objectively) and “Most Scared You’ve Ever Been by a Movie” (which is, of course, completely subjective).
My list would begin with Darby O’Gill and the Little People, and The Time Machine (1960-ish), but I was just a wee tyke.
More grown up, it would be Psycho and The Haunting, which I first for the first time saw as a young teen and was scared shitless.
As an adult: Alien.
Seriously. This movie really frightened me, unlike say, The Texas Chainsaw Massacre, or Hellraiser.
The Excorcist when I saw it home alone as a little kid.
In completely different ways and for different reasons:
Pan’s Labyrinth
Silence of the Lambs
The Blair Witch Project
Idiocracy
Event Horizon
I’m surprised to see people say this, since I find the overacting by all 3 of the principals to be absolutely hysterical. A lot of that tho is probably the Seinfeld is Unfunny effect combined with my perception of the film being colored by that of its numerous parodies (esp. the Simpsons one).
The Lost Continent (1968). A completely laughable plot-mishmash with hokey dialogue and cheap rubber monsters, not worthy of the merest passing attention of MSTK3, but I was five years old when I saw it and it was nightmare fuel. I wouldn’t swim in the sea for fear of man-eating seaweed-vines.
Easy: Play Misty For Me. Much scarier than all the horror films I’ve seen (and loved), because it’s so plausible. The very fact that it could happen (and probably does all the time) makes it vastly more scary than anything supernatural.
The Ring. I watched this movie outdoors in broad daylight on a 15" laptop screen with 4 other people and it still scared me so bad I slept with the lights on for a week because every time I turned them off, I saw Samara’s head coming out of my wall.
I’m really affected by weird/crazy eyes and that movie was full of them.
I was going to, and in fact attempted to post about a movie called “Death Smiles on a Murderer” but it disappeared so many times that I don’t feel like typing the post again. I’ll go along with *The Ring *as my second scariest of all time and definitely the scariest I’ve seen in my adult life. Holy crap, what a near perfect horror movie (if you’re into that genre). Perfect photography, perfect score. I thought the opening was brilliant in the way it kind of spoofed your typical horror movie and then turned out to be truly suspenseful and scary. I saw it on a gloomy night on my first night in San Francisco with a virtual stranger (match making through a friend) and had no idea what is was about. The “closet scene” made the whole theater shriek. I swear I saw that nasty face in my minds’ eye for months afterward. I finally bought the DVD and made myself watch that scene repeatedly in slow motion to desensitize myself to it. Today, I’ll put it on if I want something to watch while I try to take a nap. Something about the blue tint photography and the great music really relaxes me, but there was a time that film creeped me the hell out.
Paradise Lost, a documentary about three wrongly convicted and imprisoned young men, is one of the few movies that ever gave me nightmares.
Nightmare on Elm Street came out when I was in high school and it was really scary at the time. I’ve seen it again recently and it doesn’t have quite the effect that it did then.
My wife’s older brother let her watch it when she was six and I think she’s still traumatized by that. 
I did the exact same thing. I eventually had to turn on every light in the house.
Thinking back on it, I don’t know why I didn’t just turn the damn channel.
As an adult, I don’t get scared, but I do have to give Blair Witch props for giving me the willies.
I enjoy the Paranormal Activity movies for the same reason.
The whole “unseen monster” vibe is the only thing that comes close to stirring up visceral response from me.
In the theater, Jaws scared me more than any other. But I was 10 and it was the first scary movie I saw in a theater, so that was, umm, a perfect storm of sorts.
The Shining was scarier. The Thing (Kurt Russell version) not far behind.
Hafta add, also, Blair Witch Project. Even though it’s a punchline of sorts, it got me in a big way.