I do not count Berlin among my favorite destinations. I am aware of its reputation of a happening and up-and-coming place, and there are certainly more than a handful of attractions worthy of a visit in the German capital, but on balance it fails to project sufficient draw for me. A completely subjective personal opinion, mind you.
One sight that I can call a favorite in Berlin is that of the Neptune Fountain, located in a cluster of landmarks around Schlossplatz, with the dome of the Berlin Cathedral in the background.
One of the symbols of Berlin, the Brandenburg Gate, also merits inclusion. During our visit in 2005, there was an exhibition commemorating 60 years since the end of the second World War that juxtaposed the ruins of 1945 Berlin landscape with its modern lines. Taken from not the most evocative angle, but still quite powerful.
Reichstag would be another inclusion, but I fail to locate a good picture in our archives. That might be my main reason to consider visiting Berlin again.