Two of my favorites...

Both of these movies came out in the late 70's and apparently that was a very influential time for imprinting on my psyche. These, to me, are the epitome of the horror film. The encompass the supernatural, the things we can't see, the disembodied voices, the religious implications of what we consider good and evil, and the helplessness we feel when we are not in control of our surroundings.
If you haven't yet been "horrored" out, put these two on your list.