Having just returned from Warsaw, while the Westin looks nice, I have to admit that I was blown away by how nice the Meridien Briston is/was, which is located in a great location right next door to the Presidential Palace and up the street from Old Town.
The Bristol is a true grand hotel in the European sense in both style and service. I think it is far and away the nicest hotel in Warsaw, and a pretty good value compared to what the cost of a comparable hotel would be in another major European city.