My favorite horror film of all time would have to be "Repulsion". I realize Roman Polanski is a dirty word right now, (or two dirty words) but there's no denying he made some cinematic masterpieces in his time. It's one of the most disturbing and unsettling pieces of cinema ever made. Clearly a huge influence on Kubrick when he made "The Shining", but superior in every way (Both films even share the same ending - a long tracking shot into a photograph.)