You could easily spend days looking through the museums here on the island in Berlin - and I wish that I could have. Instead, I only had one day to see them all. It was a bit rushed by the end but, then again, my head was already too full by then! :)
You can buy a combo ticket for the museums and I would recommend doing that because you really want to see the range on offer - not just exhibits but also the architecture of the building.
My highlight was certainly the Pergamon and I recommend you don't miss that one, whatever you do! The New Museum is spectacular, particularly for the Egyptian collection, and I also really enjoyed the Old National Gallery - particularly the design on the building interiors.
I rushed through the Old Museum because it didn't really grab me. The Bode Museum was a surprise for me and had a lot more than I expected with a great variety - but unfortunately I got here only shortly before closing and had to just walk through quickly. I think it deserves a bit more time than many people give it.