I don't think I watch any movie more than twice a year, but these are the 10 I watch over and over. (although I've watched The Evil Dead (1) and the reboot twice this year)
- The Exorcist
- The Shining
- Evil Dead 1, 2 and 3 (I guess that's three)
- Halloween
- Poltergeist
- Scream (although not as often any more)
- Alien
- Night of the Living Dead
A couple honorable mentions that I watch about every year, but are still too new for me to have watched a ton:
- The Descent
- The Hills Have Eyes (remake)
- Saw
And a couple honorable mentions that I watched a ton as a kid, but only every few years now:
- The Birds
- The Watcher in the Woods
- Psycho