Entertainment Weekly's website has compiled a photo essay of the Scariest Movies of All Time. It seems everyone does this at one time or another, but take a look at this list - it is fairly accurate in my book. There is no #1 or anything - just a random list. To me, though, they kinda seem to go in order of scariest to least frightening.
What's your favorite? I'd probably say The Exorcist or The Shining, it's a toss up.
Oh yeah, I forgot Jaws.... I change my vote. It's Jaws!