The Halloween theme song, although I may be slightly biased, since Halloween II traumatized me as a kid. The song, therefore, is forever forged with those memories. Although it must be pointed out that all Halloween movies have a different version of the theme song, and none of the others freak me out much like the Halloween II version.
This one seems to have the more madness in it, and truly seems to match with the sense of panic and dread I felt throughout the whole film, but again might be just me.
Personally though, I find most horror soundtracks to be anything but scary. It’s often overdone, and never seems to fit the theme it should be going with. Crazy violins and little girls singing has been done so much that it feels like saying the same word over and over until it seems to lose all meaning.
That said I’d have to go with the theme song from Dawn of the Dead. It conveys a lot of the movie’s atmosphere, and that’s always important to me when watching a horror movie and trying to get into the feel of realism it may be aiming at. I also tend to like music that isn’t really music, but just a bunch of loud noises. Like during chases and stuff, although I can’t think of any direct example…