Pumpernickels is quite possibly the best deli in the area. With a NY style deli feel - and quality food - at an affordable price - its hard to beat.
They have a full range of sandwiches and salads, and cater specifically to vegetarians and vegans offering non-meat versions of many of their sandwiches.
Surprisingly, the also offer very high quality NY style pizza - which i have found to be a rarity here in DC. The place is small - there isn't a whole lot of room, but it is well worth the trip.
Its located in the Chevy Chase area of DC - very close to the border of MD and DC.