Circus Hotel. Highly recommended.
Good Location, Nice funky rooms, very friendly and helpful staff, good breakfast for 9 euro.
Stayed here for 3 nights in February. If going back to Berlin, I would definitely stay here again. From the location a few metres from the Rosenthaler Platz U-Bahn (about 45-50 mins from the airport via Alexanderplatz), to the very friendly staff we encountered from arrival and throughout the 3 nights/ 4days we had in Berlin, we really enjoyed our stay.
The lobby of the hotel is very welcoming, with nice seating, a desk with internet access and it opens into the restaurant/bar (called Fabisch) which is nicely decorated and furnished (and is much nicer than the one photo I found on the Circus Hotel or Fabisch website) . The hotel rooms are decorated in a funky style. Seemed like new when we were there. We had a double room on the 3rd floor looking into the courtyard in the centre of the hotel.Very nice room, with a decent amount of storage space, and very good shower. Bed was very comfortable too.
We went for the 9 euro breakfast buffet each morning and we also had it on the day we arrived as we arrived about 10am. A very good range of organic and very good quality foods. Juices, cereals, mueslis, breads, meats, cheeses, fruit, tea, coffee everything and in a nice restaurant setting. I would recommend the breakfast. We did not have dinner here, but on a couple of occasions, we had drinks after we arrived back from a day out and about in Berlin. The Restaurant/Bar is a nice place to hang out. There are lots of other options for bars and restaurants in the immediate area.
The location is great, and you can walk to lots of the sites of Berlin if you wish. But you are also a few metres from a very efficient public transport system with the U Bahn station which is 2 stops to Alexander Platz. Berlin is big so it is likely you will use the public transport system several times.