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I am not a professor myself, but I am going to report my observations from CS in Germany as a native speaker:

Basically, this depends on personal preference. However, there are some more and some less frequently accepted ways of addressing, so I'll compile a little list of options (centering around a fictional person named Thomas Müller):

  • Herr Professor Doktor Müller: While formally correct for most professors, this is rarely used in spoken language and should be reserved to written letters - and even to the more formal ones among these.
  • Herr Professor Müller: Usually acceptable. It's the default way of addressing a professor, and even if the professor might not prefer it, it's a safe way that won't offend anyone.
  • Professor Müller: This is not a typical (I would even say: correct) way to address someone in German. Titles such as Herr and Frau (Mr./Mrs.) are used even when the degree (or position) is indicated - the addresses Captain Meier, President Schmidt, and Doctor Huber translate to Herr Kapitän Meier, Frau Präsidentin Schmidt, and Herr Doktor Huber, respectively. (Only when talking about oneself, such as on one's academic website, Herr and Frau are dropped.)
  • Herr Doktor Müller: I have never heard about addressing a professor "only" as a doctor. The higher title always supersedes the lower one, and in this respect, Professor is definitely considered higher. Calling a professor doctor would seem quite odd to me, maybe comparable to addressing an M.Sc. holder as a B.Sc. (even though they have an M.Sc. in the same field as the B.Sc. as a direct follow-up). It feels like ignoring/neglecting some of the progress they have made in their career.
  • Herr Müller: This is very commonly used, and quite some professors seem to prefer it. I have heard various justifications, such as avoiding overly long titles, and professor being merely the position, not a part of the name. Some professors also prefer being addressed like this by their/other university staff (post-docs, doctoral candidates, ...), while expecting to be addressed as Herr Professor Müller by students.
  • Thomas Müller: Using the full name (given name + surname) is not a usual way of addressing people in German.
  • Thomas: It is rare for professors to allow this form of addressing to students, though within their own institute, first-name policies are not uncommon, especially with younger professors.

Note that for female professors, you can also use the female forms Professorin and Doktorin, although this might make your words sound even a bit more formal (as in writing).