Okay... easy call.
If you want nice rooms, central location, and spotless, new facilities then stay at the Circus II at Rosenthaler Platz (U7 line). It has a cool bar downstairs which makes meeting other travellers super easy.
If you want good rooms, okay location (Zinnowitzer Strasse on the U6 line), but a friendlier and more "party minded" staff who actually don´t mind hanging out with the guests, go for Mitte´s Backpacker. Facilities aren´t so new, but it´s got a bit more personality than Circus II. The same owners own BaxPax (Goerlitzer Bahnhof on the U1 line), which is smaller but in a cooler neighbourhood, and can be a real blast if you end up with the right combination of guests.
If you can live with a bit of grit, you can go for Lett´em Sleep. Small, rundown, but in a very cool part of town, and the staff have a good line on the Berlin underground scene. About a 10 minute walk from Eberswalder Strasse station on the U2 line.
Hope the info helps - Got any more questions
Twix
(Living in Berlin for over two years now and have worked at three of the above four hostels.)