Wow what a fun thread.
I have no single answer to this question, but I have probably seen "Se7en" more times than any other single film, so take that as you will. I often answer "Jacob's Ladder" just because I like how it mixes up real and hallucinated threats -- the brilliant use of an unreliable POV character. A lot of people short change it, I think, by using the Owl Creek Bridge ending as an excuse to neglect sorting out the intricacies of the very realistic alternate reading that much of the film carefully preserves -- the experimental drug causing hallucinations while government agents attempt to assassinate the vets as part of a cover up. OTOH, my wife thought the pace was too slow, and I guess I can see that too.
I will agree also that "Alien" is an impressively terrifying film even after all these years.