Country-wise rankings with credibility for audiences across the globe are hard to find. However, if you can get in touch with someone from Germany who has done or is planning to do an MBA, they could probably share with you a couple of rankings by credible publications.
Till then, I believe your best option would be European rankings by dependable publications like the Financial Times. While there are other reliable publications that publish MBA rankings for Europe, they are generally too short to pull out more than a couple of German schools.
The Financial Times’ Best Business Schools rankings 2019 ranked 9 business schools from Germany. Pretty much making it a Top 10 German business schools list.
Here is the list of German business schools in the order that the financial times ranked them.
FT Europe Rank: School name
9: ESMT Berlin
14: ESCP Europe
23: WHU – Otto Beisheim School of Management
24: Mannheim Business School
32: Frankfurt School of Finance and Management
57: HHL Leipzig Graduate School of Management
69: ICN Business School
74: University of Cologne
76 Tum School of Management