For me it was Event Horizon, that was scary as hell when I was a kid, I had nightmares for months.
When I was little, the Gremlins movies terrified me! Not sure why since they’re super campy, but 🤷♂️ kids are weird.
I just rewatched gremlins, and beside some jumpscares it’s pretty camp and tame.
But then the scene comes where the Santa Claus gets stuck in the chimney, discovered due to the smell. This is not shown, just a story that gets told pretty off handed.
I realized that that was the part that made the movie scary to me when younger. And it colored the rest of the movie darker, too.