apartment in Berlin

It depends on your price point and what you want to see in Berlin. For families who want to do the touristy thing, Go-Apartments Checkpoint Charlie is well priced and extremely well located. But I don't particularly get much out of that neighborhood, and would prefer the previous suggestion at Hackeshermarkt,

Last year, I rented an apartment for a week from Berlin Habitat. I rented a small studio (the "Capri") that was close to the Rosenthaler Platz u-bahn station. I loved the location and the company was very easy to work with (took credit cards, no damage deposit, etc.). I posted a review on tripadvisor.

Agree with the others. I just stayed in an apartment I found in Prenzlauer Berg on AirBnb, but your location and interests are up to you. Hackescher Markt is as good a location as any.