the Exorcist and Jaws
I still torture kids with the Jaws music every chance I get. 
The scariest movie music ever has to be the theme from Suspiria. I found a nice video but I fear it is too graphic to post.
Second I think might be The Exorcist because a lot stuff in that movie is subliminal. I saw it for the last time when they did The Version You've Never Seen around 2000 and I was impressed with how the images don't really make any sense except for the soundtrack. The first half of that movie isn't even a horror movie, it's like a very intense after school special.
Jaws, while a great movie, never really scared me. I lived near Lake Michigan and there aren't any sharks in there. But I do remember being scared that Humanoids from the Deep might have sneaked into our swimming pool during the night like the water bugs did waiting to eat us in the morning.