The most haunting music you ever heard?

You want haunting, even transporting. Find the original 1970 Exuma LP. Transcendent.

Richard Strauss "Metamorphosen" A piece he composed for strings, if I remember correctly, his response as a German, to finding out about the existence of concentration camps.

I would second Leonard Cohen's "Hallelujah". I also have the remastered Jeff Buckley version, but Cohen's original on a top set up sounds so rich and holographic and the chorus sends a chill up my spine. I would also put Peter Gabriel's "Dont give up" in the same class, with Kate Bush singing the chorus.

Oh man now it's got to be "Uncle Meat" by The mothers of invention

No one for "Tubular Bells"?