Stayed in Marriott and Renaissance Hotel in Prague(-East) in April/May this year.
Centrally located (opposite of each other, walking distance to Powder tower) and usual first-class comfort (nothing exceptional, however - the Marriott is slighty better than the Renaissance in Prague, IMO), you can try with Priceline (around 90-130 USD/night); Moevenpick Hotel (4 star in Priceline) should be very fine and in nice location as well. (Prague-West)

Check for hotelscoop.com for hotel photos of Marriott & Renaissance.