I sincerely doubt the movie itself was high-grade horror, but I saw it when I was about ten or so and it became the focus of all my worst nightmares ever since (I’m 32 now, and that hasn’t changed): Invasion of the Body Snatchers. It was a black-and-white movie, as I recall.
Were I to watch it again (or even a remake) I don’t honestly know if it would scare me now. Nightmares are *always * scarier than any movie could be. 
Highlighted by me.
I read this as ‘The Grinch’ not once, but three times.
Got 3/4 into the sentence before my mind caught up and went ‘HUH?’ … so I re-read it, got to the same point and then went ‘…wha?’.
It wasn’t until I went to quote this and had it in the editing box that I finally read it as ‘The Grudge’. Until then, I was so sure you were being funny.
Has anyone else seen Eraserhead, a David Lynch classic ?
It’s probably not ofically a horror movie but if you’re looking for something that’s scary and disgusting at once… 
No, it’s not super-bloody, not even in the same gory league as Dawn of the Dead. I found it more tragic than horrifying. The goriest bit is, oh, about 2 seconds into the movie, and then the same scene is revisited toward the end. So, if you can take the first few seconds, you’ll be good. It’s a good movie, though I wouldn’t even classify it as a horror movie.
As a pre-teen it has to be Poltergiest for me. Back in the day we didnt have cable and seeing static on the TV was something that could happen very easily late at night! I was terrifed for months that I would fall asleep with the TV on and wake up to static! Scared the hell out of me.
As a teenager, Nightmare on Elm St did it for me. I had a many a sleepness nights over that one!
As a twenty-something Se7ven did it for me, that movie was just creepy as hell.
In my thirties now, and The Ring has to be the scariest I have seen so far. That girl just freaked me out.
Another vote for The Shining.
In a simular but not as good vein I thought Scanners was rather frightening. It’s also one of those where the tension is built up with a creepy score without overly much actually happening. Apart from it’s famous scene, that is…
Mind you, this was years back. I’d probably find the film silly now.
Remains of the Day. If you are a bachelor older than 30, that movie is terrifying.
You mean Monty Python’s And Now for Something Completely Different
Stephen King’s Pet Sematary. Between the guy with the brains leaking out of his head, the crazed elder sister in the flashbacks and the gory little kid, I didn’t sleep for a week after seeing that.
I didn’t think it was particulary scary, but I felt it was a great lovecraftian horror movie. A shame that all the reviews seemed to miss the point and kept comparing it to Wes Craven’s New Nightmare.
While your point about “real” evil being scarier than “imaginary” evil is well taken, I’ve never quite been able to shake my Catholic upbringing. Though ordinarily a rational and scientifically minded person, I still believe that demons are real. That being said, horror movies do not scare me. Except for The Excoricst. That one scares the shit out of me, and always will.
Sorry, but I must vote for Psycho. It was the first film I saw that spooked me so much that I literally stayed away from showers for a while. That was after the first time I saw it. Since then, I’ve watched it at least half a dozen more times and enjoyed the masterpiece for its suspense and misdirection. Part of what made the movie particularly scary was that it COULD happen! When I saw The Exorcist, it was a fun movie, but it did not grab me the same way because I didn’t believe in satanic possession. Also, I read the book first and the movie was fairly faithful to the book, so some of the suspense was taken away. Slasher movies (like Friday the 13th, Elm Street) don’t bother me much because they’re so unbelievable and I can imagine the movie cameras filming the action.