Most people here already seem to agree on Klaus Schulze, Kraftwerk, Tangerine Dream, Autechre and Biosphere.
I also enjoy Brian Eno's ambient music, Aphex Twin (especially Selected Ambient Works series), Murcof, Four Tet, Amon Tobin, Portishead, Coil. Many of the more mainstream-sounding acts like DJ Shadow, Boards of Canada, Deadmau5, Daft Punk, Shpongle etc have some really brilliant spots in their catalogue along with (usually) some throwaways.