The most haunting music you ever heard?

Seems like it depends a lot on what "haunting" means to a person. Just off the top of my head, these are two I find haunting: Springsteen's Stolen Car and Chris Isaak's Wicked Game.

Barber's Adagio for Strings is up there...

If anyone can identify this very haunting tune, please post it, thanks.

Richard Wagner's prelude and liebestod.

"Snowed In At Wheeler Street" from Kate Bush's latest album is a current release that has that quality.
Furtwangler's recording of Tchaikovsky's "Pathetique" symphony on DG.
"Gentle On My Mind" as recorded by John Hartford in "Natural To Be Gone" also comes to mind.