I could definitely recommend the Radisson Blu, close to Aleksander Platz if you are interested in a spectacular lobby. There is a huge cylinder formed fish tank in the lobby and a lift going thru it. I would also ask for a room facing the lobby, since your view will be straight into the tank - waking up and seeing right into an aquarium. Watch out for the divers, who clean the tank though .

http://www.radissonblu.com/hotel-berlin