If you still have the choice, definitely include Dresden in your itinerary. It has been restored close to its pre-war glory and is probably amongst the most impressive cities in Germany. It has the added benefit of being a stone's throw away from Glashütte 
Food wise (been some years) I remember Weisser Hirsch, wonderful food generally and a truly sublime Engadiner Nusstorte
May also want to take a paddle steamer tour down the Elbe through Saxonian Switzerland.
Frankfurt is possibly the most boring of your choices - there is a tiny old town - and the rest is not that impressive to behold. Does have the only German skyline, though.
And home of Sinn 
Cologne and Aachen are nice with plenty to see (but been too long ago for me to give any useful advice).
Berlin is one of the 'freshest' capitals in Europe. The East-West mix makes it relatively exciting and while not haute cuisine, one needs to have a kebab there - supposedly the best ones outside of Turkey 
Hope this helps a little bit!