Best horror? Define best.
Personally, for me, it's always about these:
1. The original, black and white, The Haunting. Scares the tar out of you without anything actually happening. The absolute pinnacle of atmospheric scares.
2. Night of the Living Dead. It's not about the zombies, folks.
3. John Carpenter's The Thing and the Hudson Hawk Thing From Another World back to back. Great movies and still so tense that you could imagine the sound of guitar strings snapping under the strain.
4. House on Haunted Hill. The original Vincent Price version. Not because it was particularly scary, mind, but because it was one that terrified me as a child and to this day gives me a guilty little thrill every year.