I like the Waldorf Astoria and the Hilton Berlin.
Hilton Berlin is very well located for all the usual sights - it's on Gendarmenmarkt. Next to the metro stop Stadt Mitte.
The Waldorf Astoria has easy access from the airport (Tegel) with a fast public bus and is right next to a metro stop and a train station - Zoo Garten. To go to Potsdam is about 25 minutes on the regional trains from the station. It's in the Kurfurstendam area with a lot of shops and department stores (including Ka-De-We ). And it has a very nice Spa Area.