There are several famous universities, but we don't have a clear ranking. What would you like to study? Would you prefer to live in a big city or a small town? Do you want to study in Germany as an exchange student or do you want to get a degree? Are you fluent in German or do you want to study in a program taught in English? These are all questions that should be considered rather than how famous a university is.

There is historical fame, like Heidelberg's fame for being very old, or Wittenberg's fame for being the university of Martin Luther.

And there is the current reputation of a university among employers. Which of course is much more relevant. In engineering, I know TU München and RWTH Aachen are well-reputed...

There are rankings for particular subjects and degrees. Like bachelor degrees in Informatik (computer science) at Fachhochschulen, or university Masters in architecture. I would certainly take a look at them as a rough indicator of quality.