Hans-Joachim Roedelius. He was in Kluster/Cluster, Harmonia, etc. His solo work is great. I like the early stuff and the more atmospheric later stuff.
www.roedelius.com
Asmus Tietchens. Mostly all solo work. Worked with a lot of key figures though, like conny plank in a one time deal as a band called Liliental. His solo work is great if you like sound design, conceptual stuff, and atmospherics. Really sophisticated.
www.tietchens.de
Sound samples are around, good luck finding them. Youtube has a really nice tietchens track:
http://youtube.com/watch?v=cC217zYPOrk
enjoy.