Depending on your frame of mind, anything by Lustmord. Lustmord is pretty much the God of dark ambient. He is easily one of my favorite musicians ever. I never tire of listening to his music.
http://www.youtube.com/watch?v=bRVNvztrlR8
http://www.youtube.com/watch?v=OXfjQqM17NE
http://www.youtube.com/watch?v=ZD4H-E0UT0Q
http://www.youtube.com/watch?v=bpg03048Yow