What kind of ambient are we talking here? Long slow drones and background noises, or more musical pulsing ambient?
Or even, the more traditional techno style ambient?
I highly reccomend Visit Venus, and Entheogenic if its that kind of thing. Very chilled out.
Also, for long drawn out stuff, and slightly more worldy stuff, Robert Rich's collaboration with Alo Die is awesome.
Also, for the most ambient of the ambient, you can find it all here, with reviews, categorization and sound samples:
Ambience by Sound :: Ambience for the Masses