The two scariest movies I've seen; Alien and Communion.
As for Alien, it was a pretty singular achievement too. The second movie was a great action flic, but lost the horror element. The third movie stunk and the fourth was not only horrid but pure feminist pablum. Yes the whole series had an underlying feminist angle, but the fourth movie decides to replace action and plot with an ideological hammer. The same problem, interestingly enough, that Buffy the Vampire Slayer had with its 7th, and final, season.
As for Communion, I'd read far too many alien abduction type stories when I was in junior high (they had a ton in the school library for some reason) and the movie just plain struck a nerve.
Oh, and as for Hellraiser, IMO, was never that scary. The better Hellraiser movies are interesting, while the worst just gross, but I can't say any of the movies induced fear...
Did you know when Alien was first written Ripley was a male character?
read the book "Jaws" while on my sailboat in the Carib; didn't get in the water for a week...
read "the exorcist" in a snow bound cabin high up in the Sierras, no one around for miles....wind blowing the limbs of trees against the cabin, the only contact with people the radio...darker than hell...... didn't sleep well for a week.
Yep, you guessed it, I read "The Shining" here at the ranch during the long winter days....Man, did this old house have a lot of creaks and groans...