I can highly recommend CALMA Berlin Mitte, perfect location, right in heart of Berlin, you can easily walk everywhere, plenty of public transport options. Less than 15 minute walk from Museum Island and Alexander Platz, really close to main train station, and in the close vicinity of this place (just a street away) lies the greatest discovery of this trip - Samurai Museum - one of the best museums I've been too, not only for Japanese culture fans!