You cannot beat Marcel's if price is not an issue.

If you're not averse to driving to the D.C. suburbs (Bethesda, MD to be precise), I'd highly recommend Black's Bar and Kitchen. They have a myriad brunch food options and offer bottomless brunches (and even bottomless champagne brunches) for reasonable prices, especially considering the high quality, organic fare on offer (including an excellent raw bar).

at the Washington Harbour serves a terrific brunch if you happen to be near K Street in Georgetown.

Legendary DC brunch at the Four Seasons in Georgetown, but it isn't cheap.