Where to learn German "best" depends on your level and goal.
If you plan to learn German or become an advanced translator, Beiwai is the first choice. If you want to be a "master" of German studies, you'd better go to Peking University. If you study German just to go abroad, Beijing Foreign Studies University (Goethe Institute) and Tongji University are all good. In contrast, it is easier to get advice from famous teachers who can benefit you for life at Beijing Foreign Studies University, while Tongji is more popular and more suitable for students who are eager to take the postgraduate entrance examination. Tongji's advantage also lies in that no one in China can compare with it in training engineering German talents, which can be seen from the fact that most of the German Chancellor's scholarships are spent in Tongji.