These are not my pictures, they are from the architect's site, who did the remodelling. I just found the link and posted it, because there was some interest expressed here.
Garlstedter Heide (near Bremen, btw) was built in 1964 and is widely considered Bernhard von Limburger's masterpiece. It supplants an older 9-holer in the racetrack, which dates back to the very late 19th century and is one of the oldest courses in Germany. I need to play this course myself after the restoration, but have heard great things.
The next contender would be Hamburg-Falkenstein, originally a Colt/Alison/Morrison design from the late 1920s, but redone by von Limburger in the 1960s. Basically, von Limburger had his hands on most great German courses, he built almost 10% of today's 650 courses himself.
The classic in Düsseldorf, that was mentioned, is probably Hubbelrath. Yep, you guessed it, von Limburger, 1961. Generally, you can say that von Limburger is Germany's H. S. Colt - very similar parkland style.
Bad Ems is a Simpson design and I like it a lot, but not sure if I love it. I am in that area frequently and will provide a detailed report with photos. But the season is ending here, so it may not be this year.
Finally, Frankfurter. It is not my favorite course and I live there. Too many trees and a bit repetetive. Also Colt/Alison/Morrison late 1920s and, of course, also redone by von Limburger in the 1960s. I may do it injustice, will play there again. But I much prefer nearby Hanau-Wilhelmsbad.
Ulrich