Tengelmann carries a wide range wines at reasonable prices. I'd go so far as to say that, if there is one of the larger branches in your city, you could make it your sole source for wine and, unless your taste ran to the exotic you would never need to feel that you had a restricted choice.

In Munich, the branch at the Stachus Einkaufzentrum is well worth a visit for its impressive range

My one reservation is that some of their middle range wines, those around the Euro5-7 mark, are no better than the wines that they sell at the Euro3-4 price point.