In Arlington, Smokecraft is solid. I agree with the other comments....after smoking my own brisket, I find it incredibly hard to find good bbq around DC. Even when I go to Texas, I get underwhelmed by some of the briskets.
For the best pit beef, I'm a fan of Pioneer near Catonsville. I did a pit beef crawl around the suburbs of Baltimore after seeing spots written up in the Washington Post, and that was my clear winner. Chaps is good too, but it has become too commercial as they've expanded.