As someone who lives in Warsaw, and who has tried most of the hotels at one time or other, my strong recommendation would be for Hotel Verte - it's a relatively new Marriott property in a perfect location for visitors.
It's on Podwale, which puts you right in the heart of the Old Town, and the property is superb. It's a converted palace and for me it's markedly nicer than the Bristol, which has legacy rooms that feel dated. It's also usually much better value.

Last year you could get regular points redemptions here for 20K, which is crazy good value. They also have a really nice area for breakfast. The property is relatively small so there's also a bit of a boutique feel to it, but unlike many boutique hotels you don't end up with a room without any desk that's impractical for doing work in.