I would like to nominate
The Haunting (1963)
"Dr. Markway, doing research to prove the existence of ghosts, investigates Hill House, a large, eerie mansion with a lurid history of violent death and insanity. With him are the skeptical young Luke, who stands to inherit the house, the mysterious and clairvoyant Theodora and the insecure Eleanor, whose psychic abilities make her feel somehow attuned to whatever spirits inhabit the old mansion. As time goes by it becomes obvious that they have gotten more than they bargained for as the ghostly presence in the house manifests itself in horrific and deadly ways."
It is supposedly the world's spookiest movie, yet I haven't seen it! It's something I'd like to remedy one of these days.
