The Hitcher is a great suspense ride....damn....Rutger Hauer is SO FUCKING creepy in that.
Anything by George A. Romero like Night of the Living Dead, Dawn of the Dead, Day of the Dead (and some will disagree) Land of the Dead, Martin is also really good.
Some old monster films like THEM! and Creature from the Black Lagoon.
Psycho and other Hitchcock movies.
R-Point is a really good Korean horror movie.
ANYTHING by Lucio Fulci, one of the great Italian horror directors. ZOMBI 2 is a great one.
Cannibal Holocaust will make you sick to your stomach. But its a good watch.