Horror: 1. Dawn Of The Dead 2. Halloween 3. A Nightmare On Elm St. 4. Psycho 5. The Texas Chainsaw Massacre Honorable mentions: The Ring, Drag Me To Hell, 28 Days Later, The Blair Witch Project, The Shinning, The Descent Sci-Fi horror? 1. Alien/Aliens 2. The Thing 3. The Mist 4. Pitch Black 5. The Fly
1. Carpenter's The Thing 2. The Exorcist 3. Jaws 4. Psycho 5. Night of the Living Dead I also have a soft spot for Carpenter's original The Fog.
That's basically my one and only threshold for determining a horror film. "Am I scared? Yep, it's horror."
Duel is a fantastic movie. Its more of a thriller, but one of the 10 best thrillers ever made. I think its Spielberg's second best movie behind Jaws.