I second the vote for the JW Marriott. It was very nice, well-located, and close to the Metro. We were just there in June. TIP: I was there with my friend and our three daughters, and we discovered that many of the food prices (especially dessert) were more reasonable from room service than they were in the cafe in the lobby. But across the street, there is the Reagan Trade Center, and they have a great food court. We ate there the most during our stay.
The only negative was the elevators were real slow on Sunday morning when we, and apparently the rest of the hotel, were all checking out at the same time. Some people waited close to half an hour to get an open elevator!
Have fun!