In our experience, avoid Europcar wherever possible. We (and our customers) have had nothing but issues with them. Hertz, Avis, National, Enterprise and Sixt are all fine in Germany. Sixt often has very nice cars, but then again you can rent higher-end vehicles with any of the companies depending on which class you choose. You're not going going to get a powerful 6-cyl Mercedes if you reserve and economy car obviously.