I have never been to Köln (Cologne) but if I had to pick a German city to visit again I'd probably hit Leipzig or Berlin. I found Berlin to be much more interesting than Dresden and I didn't really find anything that I really loved in Dresden. Although Germany won their second handball game against France in the Handball World Cup right when the traffic jam we were in for an hour and a half magically dissappeared when we were driving to Dresden so maybe I shouldn't be too hard on Dresden.

Leipzig was a great stop and I was really dissappointed that all I got to do was sit on a bus and see the sites. I'd definately reccomend Leipzig.

Berlin is also a great choice. but I'm rushed so I can't write more.