The supermarket that I shop at, Alnatura is part of a nationwide organic chain with, I think,…read morearound 45 stores across Germany. The selection here is great for an organic place, its just as big as some EDEKA in Hamburg.
What I like the most about this place is their in-house brand, of which they have a very good selection of products they sell under their own Alnatura name. The prices on these products are generally very good- around or less than what you would pay for a non-organic name brand except, well, organic ;). Their organic cheese selection is good (I recommend the Demeter Bergkäse) and they always have three or four cheeses that are on sale.
The one thing that can get expensive here is some non-house brand things as well as fruits and vegetables- I generally only get what I really need and try to get the rest of my veggies at one of the organic open-air markets with regional producers, which usually ends up saving a few Euros (like the one on the Lange Reihe on Fridays).