In no particular order, here is a list of my favorite restaurants in Washington, DC -
Charlie Palmer Steak is located in the heart of Washington, DC (right across from the Capitol building) and is frequently used as a venue for Members' meetings, events, and fundraisers. The building is beautiful, the food is delicious (but expensive), and they have a nice wine selection.
From the outside, the Tune Inn doesn't look like much (it is a certified dive bar), but their food selection is great and affordable - especially the breakfast (and bloody mary's). This restaurant is also located on Capitol Hill and is frequented by Hill staffers and the occasional Member.
When I was an intern and lived on the Hill, I would visit Kyoto Sushi multiple times per week. To sum it up, it's casual, friendly, and fast - and the sushi is fresh and affordable.