Some other thoughts on how to choose YOUR "must see" sites in Berlin. What have you seen in Europe before? How much do you know of European/German history?
My husband has seen Versailles only a year before we went to Berlin and wasn't really that impressed by Potsdam. I've seen many palaces and castles as well but very much enjoyed the atmoshphere of the quiet winter landscape in the park.
The German History Museum is great if you have no background knowledge of European history. A great overview from the stone age to the 20th century. But a lot of that kind of history is no different from other history museums in Europe. Personally, I enjoyed most the exhibits for the 20th century which wasn't really what my husband was interested in and he found the time up to 1800 very repetitive compared to what he's seen in other places. And due to lack of German language skills he couldn't listen to the speech the leader of the socialdemocrat party gave in 1933 after Hitler enacted the "Gleichschaltungsgesetz" that basically ended any political opposition in the country. That was my personal highlight of the entire museum and for that alone I'm glad we went. (We ended up in there because the line-ups at the Pergamon) But there are English descriptions and audio-guides available.
The Pergamon has exhibits you won't find anywhere in the world. Very unique and impressive, can't imagine anyone who would not count it among their favourite sites in Berlin - our kids put it quite on top of their list as well.
Similar with the Reichstag - its history and architecture, especially after the restaurations by Norman Foster, are very unique. It is indeed "living history"
Gemaeldegalerie is great if you're into art. Would have loved to see it and am somewhat disappointed that I didn't have time. But again, after spending several days in the Louvre a year earlier and in other art museums throughout Europe there was no way I could have dragged my husband and kids in.