Ghostbusters 2 - that painting and the pink goo gave me nightmares for weeks.

Independence Day: The dissection scene

They: The whole damned movie

The Thing messed me up as a 14 year old.

Pet Cemetery, kid getting hit by the semi

Kentucky fried movie

The Gate

Aliens. I was terrified of facehuggers being under my bed for a decade. Not so much anymore.

It's never an entire movie, it's a scene here and there.
Like in The Exorcist, when they showed it on network television back in the late-70s it must have been, the CBS Saturday Night Movie or something like that, "viewer discretion is advised".
Anyway... clicking channels, I stumbled upon a moment during the ritual itself, with the girl in silhouette on her knees, arms towards the ceiling, the demon Pazuzu behind her. That screwed up many a night afterwards.

As a young adult, another scene that fucked with my head for many a night was the grainy dream transmission, with the faint audio covered in static noise, from John Carpenter's "Prince Of Darkness".

Now I'm gonna flip the concept on its' head and tell you what film cured my fears of the dark at the time. Martin Scorsese's "The Last Temptation Of Christ".

killer klowns from outer space

A Nightmare on Elm Street. I think I was ten? That was not a good night's rest.

I was a late teenager when it came out, but I got incredibly horrified once I saw the poster for My Little Pony: Equestria Girls. Seriously, who knew turning ponies into humanoids could end up looking so uncanny?

Brain Dead was plenty disturbing to me when I was a kid. Still one of my favourite movies and now I understand that it's a hilarious comedy, not just a gross out fest although there is plenty of that too lol

There were a couple for me:

I was young when Alien came out, but my grandfather had it on VHS soon after. Ermagherd. Still one of my favourites films.

Xtro.

But there's one I'll never forget. A woman on a boat, with a metal bucket tied to her belly, with a heat source underneath it, that contained a rat. The rat would do anything to escape the heat... and the film showed the process. I was probably 10 at the time, and it's an image/predicament I'll never forget. No idea what the film was called.

Plague Dogs

...it's a cartoon

Saving Private ryan. I was 11 or 12. The slow knife scene hasn't left my mind till this day.

Casino.

That scene at the end with the bats... Still haunts me to this day.

Mine would have to be Hollow man (2000) it was on TV at the time and for some reason as a kid watching the shifting both the animal and the human scene where they turn invisible where the skin disappeared then his muscles and veins, was absolutely terrifying, I don't remember well but my parents said I wouldn't let them sleep for weeks.

The first Friday the thirteenth when i was 15 or so. My parents didn't even allow me to watch the news or any movies aside from a few cartoons, so i was shocked.

Khilona bana khalnayak. Tatya bichchhu. Om bhatt swaha!

The Silver Bullet. That Werewolf scared me soooo much as a kid.

The Blob (80s version), The Thing, and House. The first two still hold up really well but the third one I rewatched as an adult and it was so stupid I was embarrassed I had been so terrified of it as a little kid.

Candyman.

Holy fucking shit, saw it when I was maybe ten years old. I didn’t go into a bathroom for weeks after that, I peed outside against a wall. There was absolutely no way I was going in any room with a mirror.