If someone visiting Berlin I would highly recommend to stay at Ilia's and Olga's apartment. It is absolutely amazing. If you are one, who is really into arts, you would really love it. The whole place has Berlin's unique atmosphere. The walls are decorated with paintings, and pictures and the flat has a wonderful catchy vibe. It will tune you to the city. Location-wise, it is not in the "inner-inner" city, but that is what makes it the best. You can sleep well, without disturbed by noises, and when you wake up you find yourself in Tiergarten, with full of trees and fresh air. There are a couple of really good places to grab a beer, or a refreshing breakfast. If you consider always staying close to the inner city, hop on a bike, maximum 10 minutes.