It's hard to find fault with Hotel Otto (where I stayed for three nights recently, while visiting friends in Berlin). The hotel is well situated close to the heart of west Berlin and many transport links. The hotel is simply and tastefully decorated. The rooms are kept very clean. The bed was very comfortable (one small gripe - the pillows are not the best, more like weird-shaped cushions than pillows).
The very best thing about this hotel? The staff. From receptionists, to breakfast staff, to cleaners - every single person I met in this hotel was friendly, smiling and helpful at all times. The reception staff went out of their way to help when my friend's bags were lost by her airline (my friend wasn't even a guest at this hotel!). We were bowled over by the great service, and thank them very much.
Breakfast on the rooftop was fantastic. It's pricey (and there are nearby cafes, which I didn't sample, if you don't want to pay E15), but it's extremely tasty and there is plenty of choice. The croissants and breads were warm and fresh, and everything was topped up when supplies ran low.
My only small grumbles... the walls weren't completely soundproof, and I was woken early on my last morning by noise from my neighbours. And the pillows were not very comfortable.
However, I will definitely be staying at Hotel Otto again on return visits to Berlin. And I have already recommended this hotel to several people I have spoken to since returning. This is one of the best hotels I have ever stayed in.