I think scary is a personal thing but I find that it is the older classics that do it the best. Most have been mentioned already but it is the distinct lack of high tech gadgetry and the need to use the imagination that made these more potent than they actually were. They used suspense and cheesy music to build to terror as opposed to blood and guts and decapitation to drive the point across. Masters of horror in the persons of Alfred Hitchcock, Bella Legosi and Boris Karloff knew how to do it and although with age you have to watch them and wonder why they scared the bejeezus out of you at the time, they are still well worth watching. When I was a kid, Frankenstein sent me under the theatre sear when the monster slowly reached up and ripped off the dressing to finally show its horrible face that to day, by comparison, is more cartoonish than horror.
Ah, the joys of Hallowe'en
