Something I decided to do for fun. My previous list, which reached over 300 titles, has been archived. As far as what constitutes horror, YMMV; as much as I love everything in the top ten, it'd be easy to force some of them off just by letting in, say, F.W. Murnau's Faust, Elem Climov's Come and See, Michael Tolkin's The Rapture, and Jean-Luc Godard's Weekend (and that's just getting started).
Honorable mentions (films previously included, now bumped): Seven (1995, David Fincher), Opera (1987, Dario Argento)