Though I can easily see myself returning to your question over and over because there is so much movie music out there that I enjoy, these three came to my mind first:
"Prelude" from Bernard Herrmann's score for "Psycho"
"Ave Satani" from Jerrry Goldsmith's score for "The Omen"
"Quilting Theme" from Thomas Newman's score for "How To Make an American Quilt" ( I got 'outvoted' on what movie we were going to see that night -- and I ended up liking this movie more than any of my friends who wanted to see it, ha; this music is so lovely to me)