My vote would be for the Renaissance DC (Not the M Street) for the best mix of a nice facility, great service, and an outstanding location. My second choice would be the Metro Center Marriott, and my third choice would be the JW.

FWIW, I've spent a LOT of nights in DC at almost all of the FS properties. And I've been a tourist, a business traveler, and a local on weekend getaways. The places I listed consistently offerered the best overall product for the money.