Ambient is magnificient, the most important thing to me musically (music is already the most important thing, so it's at the top of the top).
Some very good pieces:
Michael Brook's album Hybrid (made with Brian Eno and Daniel Lanois), very fascinating piece towards the end of the album called Earth Floor (very good album all throughout, you can listen to it all in this video):
http://www.youtube.com/watch?feature=player_detailpage&v=VlGFOZy-D1s#t=1933s
Gas (Wolgang Voigt), who's made some of the most amazing loopy, trippy stuff. Listen deep to this one, there's so much stuff in the backround you can get lost in the maze of sound:
http://www.youtube.com/watch?v=fkiqIY4La-k
Ulf Lohmann. Brilliant, just brilliant:
http://www.youtube.com/watch?v=M_yCHBInuCs
(from my own youtube channel)
So very calming, it can take all stress away immediately
Krill.Minima - Submarine poetry:
http://www.youtube.com/watch?v=S1NwPZ9tyt4
Vladislav Delay. This is a bit more dubby, deeper stuff. Might be a bit hard to get into at first but some of the most breathtaking musical experiences I've had listening to this one:
http://www.youtube.com/watch?v=OQZgDEnLVjw
And finally, the one piece I've listened to more than anything in my whole life (when I was 15 I went to London for a week and every single day I listened to this one on repeat, constantly.. Left a very weird image in my memory of the journey):
Brian Eno - Signals:
http://www.youtube.com/watch?v=NYEJB8XMvZI