Based in the choices listed, this was not as easy decision. I went with Amityville Horror as it's based on a true story. Of course, so was The Entity, but Amityville got my vote. The Omen freaked me out. I saw the Exorcist long after it had been introduced, and the special effects seemed hokey, but had I seen it during it's hayday, I might have voted for it.
My all-time favorite is the 1968 original, Night of the Living Dead. Scary as hell when I first saw it in the early 70s. Still pretty good today.