Well, strictly speaking, Zelenka was not German, he was a Czech Bohemian (which means that he did not come from any of the states of the Holy Roman Empire of German nation either because Bohemia, while part of the territories ruled over by the Emperor in Vienna who also presided over the German states, was not part of that), but he spent most of his artistic life in Dresden, and he also studied in Vienna, so his work can not be looked at separated from that of his German contemporaries and colleagues.
Zelenka composed a number of highly interesting works. Among my favorites are the trio sonatas for oboes and basso continuo which I have all played a number of times. They have also been recorded numerous times. I particularly like these:
The one on the left features Goritzki and Glaetzner, the one one the right Holliger and Bourgue. Both are on modern instruments, Holliger/Bourgue more "modern" in style while Goritzki/Glaetzner incorporate "HIP" elements in a very intelligent and stylish way in the way it can also be heard from the Neues Bachisches Collegium Musicum Leipzig under Pommer from this period (and the accompanying musicians come from that environment, too).
Oboe players love these works because they are very inventive and demanding in their use of the instrument. Zelenka was a bass (or violone as it was usually called then) player and that is reflected in the interesting bass parts, too.
Dresden, then as now was one of the most important musical centers in Germany, musicians enjoyed excellent working conditions and the Hofkapelle (which still exists today in the form of the Staatskapelle) was renowned all over Europe. Vivaldi also wrote concertos for them. Zelenka's colleague, Johann Adolf Hasse who I believe has not been mentioned here yet, was one of the most celebrated composer of operas in the Italian style during that time.Here is an interesting article about music in Dresden in that period.
Hasse had succeeded Heinichen as Hofkapellmeister. These are great albums of their music:
Other important German composers of the time not yet mentioned were Johann Joachim Quantz and Carl Heinrich Graun who both worked at the court of the king of Prussia, Frederick II ("the Great") who played the flute and indeed also composed himself. We are getting more into the late baroque era here, but there are no strict "borders" between "high baroque" and later styles anyway.