Which I think is the opposite of what the OP was going for by nominating Fail Safe as the “scariest.”
Glad you mentioned that one. I must’ve been 5 or 6 when the original Thing showed up on our tv and I made the mistake of watching it alone. Big mistake.
IMHO (in my head cannon) there’s two separate categories. There’s those movies, whether The Exorcist, A Nightmare on Elm Street, The Conjuring, Friday the 13th, Annabelle, etc. which are more in the realm of fantasy about monsters, spirits, ghosts, demons, etc. I would go so far as to include movies that depend on “jump scares” in this category.
Then there’s movies that cause me to feel more a feeling of dread rather than the “scary” that the above movies go for. Whether that be the nuclear war movies mentioned by several posters, or dystopias featured in movies like The Purge or shows like Handmaid’s Tale, the feeling they produce in me is definitely different than the those in the category I mentioned above. And not in a pleasant way.
I generally refuse to watch movies in this second category, although somehow I did end up watching one recently thinking that it was actually a movie from the first category. Clown in a Cornfield, had it not mixed in some pop up scares and such, is a contender, since the villains are a bunch of middle aged corrupt MAGA type small town business men and cops who are tired that their children are becoming corrupted by the liberal outside world.
Sorry, I didn’t make myself clear. Zombieland is the funniest horror film I’ve ever seen. The fact that it’s (nominally) a horror movie is what qualifies it as belonging on a list of the “scariest.”
HOMER SIMPSON: You know that movie, Young Frankenstein? Scared the hell out of me!
MEL BROOKS: Um, thanks.
No, I meant scariest movie. I chose Fail Safe because I, personally, found it scary because, as I said upthread, it could have happened. Sure, there are scary horror movies; but for me, reality is more frightening. (I had a friend who used to say of horror movie creatures, ‘Eh, you’re too ugly to be scary.’)
This was my experience with The Exorcist. I was in college, early 80s. Both of my roommates were gone for the weekend. I got home from work around 11:30pm, I wasn’t tired and wanted to go do something, but it was too late to be calling around. I saw that The Exorcist was playing as the midnight movie at the student union that weekend, and I hadn’t ever seen it, so I walked over and saw it by myself.
OK, so you know the scene early in the movie, before the possession, when they think they are hearing rats up in the attic? Well, I’m back home after the show, already having a hard time getting to sleep because I was a little freaked out by the movie, when my roommate’s cat starts scratching at my door. 
I don’t think I’ve ever actually seen it since then. There have been a few times I’ve wanted to pull it up on streaming, but my wife absolutely will not watch it.
The X-Files - Home left me disturbed, unnerved, shook.
Deliver Us From Evil, 2006 Documentary about a child raping priest, retired and living on his pension in Ireland. The interview with him is possibly the most horrific thing I’ve ever seen, because he simply seemed unable to understand that he had done anything wrong. The idea that normal seeming people could be walking around, functioning in society, and yet be a complete moral void- it’s frightening in a way that no fictional monster could ever be.
It was for me, and I was in my 30s at the time. I was standing a lone night watch, on call for urgent maintenance issues. The building was completely empty but for me, and the bunk room had a TV to keep things from being completely boring.
Anyway, I came back from a call-out and thought “Let’s see if there’s anything on the toob.” I turned on the TV and there are these two people walking through a cemetery and arguing. In other words, the opening scenes of NOTLD. But I had no idea what I was watching and had never heard of the movie, so as it started moving along I was like “HOLY FUCKING SHIT, WHAT IS THIS?!” Didn’t sleep much that night.
That ain’t nuthin’. Watch Imprint from Masters of Horror, directed by Takashi Miike.
Even Showtime refused to show it.
If you are a bachelor older than 30, Remains of the Day is terrifying.
The most infuriating, soul-crushing film I have ever seen.
Helter Skelter. Not the movie, but that it really happened.
A friend of mine, who had never lived alone-- always at home or a dorm, was housesitting for a professor over winter break.
One evening, she went looking over his books for something to read, and found a book she’d always been curious about…
…Helter Skelter.
I would say it is a tie between Bob Roberts and Wag the Dog. Scary in that I can see them easily happening (recently, I think they ARE happening!). When they were new, seeing how easy it could be to manipulate the masses scared me deeply.
Third might be The Parallax View, and whenever they talk about trump’s “assassination attempt”, I think of this movie.
One of the few horror movies that actually scared me at a fundamental level. The entirety of the Ring video was a bonus feature of the DVD. I would not watch it!
“Seven days.” click
John Carpenter’s “The Thing”. Ones that scared me as a kid: OG “Invasion of the Body Snatchers” and “Fiend Without a Face”.
This might make for another interesting thread - films that are cautionary tales.
I think Remains of the Day is a beautiful, but sad film. Similar to Brokeback Mountain in a way. And while I actually am a bachelor around Mr. Stevens’ age, I’m comfortable that I haven’t made similar mistakes (rather, I’ve made my own original mistakes).
My entry in the cautionary tales genre would be The Caine Mutiny. I never want to be a Captain Queeg.
Any adaptation of Madame Bovary.
Welcome to my world. My teenage daughter LOVES scary movies. She is constantly asking me for my scariest movies I have seen and what are good ones for us to watch. I actually initially opened this thread looking for any new options. There have been several movies that she had that exact reaction to me being scared when I watched them. (The ones I can remember that reaction in particular were “The Ring” and “Poltergeist”.)
The funniest thing about this is that the stinker almost never flinches at the jump scares while I generally do. So now I get teased by my daughter while we watch the movies. I just tease her back on other things and we have a good time with it.