Depends on which vitage you're thinking. Dürkopp was top of the line for racers before and after the war. For utility bikes, before the war a Brennabor was the thing to get (my grandpa told me).
Diamant was a benchmark, too.

After the war, many companies made fine racers - Stollenwerk, Express, Patria, Adler, Bauer, Panther, Rabeneick, Enik, Schauff, Kotter etc. A lot of interesting small frame builders built nice bikes from the 50ies on - Rickert, Flema, Altinger, Redl, Pyttel and numoerous others. Diamant continued to build racers in the DDR limited by the usual communist-stlye lack of everything, so Diamants between 1945 and 1989 are interesting, but typically very crappy. Textima, also in eastern germany, supplied the top athletes with some groundbreaking material mostly for track use, and are highly sought after.

For availability in north america i'm no expert. There were at least some Dürkopps and also Rickerts being brought over the pond, Puch/Austro Daimler has been mentioned before. (I'd have a close look at any Puch buying, conditions in the factory were so messed-up at times that someone wrote a book about it. They still are absolutely iconic for any austrian).

The german equivalent of the "bike boom" half racers are usually german only in name (manufactured in cheap-labour eastern europe) and suffer from the same flaws as their siblings from other countries. I'd stay away from those.