…those you saw as a preteen, found barely plausible and chattered about the next day at school
So for me The Omen and The Exorcist.
Special mention for the BOOK Amityville Horror. Unfortunately it didn’t translate to film well.
Checked 3 for me there. Also the Legend of Hell House is a candidate.
Special mention for giving me nightmares for years, I saw Carnival of Souls as a 4 year old. Not a good idea.
Nice call on “Legend if Hell House”. I saw it as an adult, but had I seen it at age 9…it would have terrified me.
Another one I remember that fits the bill is “Helter Skelter”. You’ve got the tattered paperback being passed around and by the time that’s absorbed, the mini-series comes out.
Frankenstein 1970 (which was, in fact, released in 1958). I was seven or eight years old, and having a sleepover at my cousin’s house (they lived three blocks away from us); for whatever reason, my aunt let us watch the first part of the movie before we went to bed.
The last scene we saw, before we had to turn it off, was of Dr. Frankenstein assembling his monster. He took the heart out of a victim, and there was a close-up of his hands, with the heart in them. In retrospect, it was probably a wad of cotton batting, soaked in corn syrup, or something like that, and it was in black-and-white, but that image was seared into my little brain, and it haunted me for years. Even now, close to 50 years later, I can still see it in my mind’s eye. 
Jaws. I was 13, and it’s still the scariest movie I’ve ever seen.
The Hellraiser movies.
I’m a bit younger and I do recall looking behind us several times on the drive home to make sure we weren’t being followed by a giant shark. That was a scary but great movie.
I share the OP’s hypothesis. When I was very small, not quite five, my parents took me to see Poltergeist. It scared the ever-living hell out of me and I had nightmares for a year.
As a result I wasn’t allowed to watch most horror movies until I was in high school, which means I missed out on many of the horror movies my contemporaries to this day claim to be the scariest movies they’ve ever seen until I was much older than when they saw them. And these “scary” movies are not. The Fog? Jaws? The Changeling? The Shining? Not very scary, and with the exception of Jaws, actually pretty boring.
Having rewatched it as an adult, Poltergeist isn’t really that either.