Here are 12 (mostly inexpensive) suggestions for the DC area. If you truly want to taste the best of DC, I would go with our strengths – seafood (especially crab) and ethnic foods. Our proximity to the Chesapeake Bay and the Atlantic makes seafood a good bet, and many of our residents of foreign origin have brought the best of their respective cuisines with them. Good seafood will usually cost you (with Cameron’s being a noted, and welcome, exception), but the ethnic restaurants are usually some of your best bargains.
Cameron’s Seafood Market – a very good place to get fresh, raw or cooked seafood at a relative bargain.
Full Kee – Chinese. Excellent meal-in-a-bowl soups and roasted meats.
Ben’s Chili Bowl – Very good chili-dogs, -burgers and -half smokes.
Mixtec – Mexican. Great tortas (Mexican subs).
C.F. Folks – Lunch only. Delicious and varied daily specials, including (in season) a very good, and bargain priced, crabcake sandwich. http://www.cffolksrestaurant.com
Pizzeria Paradiso – The best pizza that I’ve found in DC.
Ollie’s Trolley – Used to be part of a national chain of burger joints, but they are now independently owned and operated. The ‘burgers are big and look good, but, eating very little beef, I must admit that I haven’t tried one. This place gets recommended because of their "trolley" decor and their surprisingly good crabcake sandwich platter, which consists of a very good crabcake, decent fries and a dollop of cole slaw, for 5 or 6 dollars.
Bethesda Crab House – Assemble a mound of steamed, well-seasoned blue crabs, a paper-covered table, a wooden mallet, some ears of fresh corn, a cold brew and some good friends, and you have the quintessential Chesapeake Bay-area dining experience.
Jerry’s Seafood Restaurant – Expensive, but if the comparably-priced Crisfield’s qualifies as roadfood, so does Jerry’s, which I think is better. Crab is the thing to order here, in any of its incarnations (cakes, imperial, soup, bisque, "Bomb," "Baby Bomb"). http://www.jerrys-seafood.com
Chinatown Express – The chef standing in the front window making noodles lets you know what to order here. The noodle-based dishes and the dumplings are very good. Especially noteworthy are the Shanghai soup dumplings, which are dumplings filled with pork and broth. They are referred to simply on the menu as pork dumplings (8) and make a great lunch.
You mentioned possibly eating at a southern, or "Soul Food" place. Two good choices of this type are Adelis, which is a buffet featuring a large selection of dishes prepared Southern-style, including delicious fried chicken, oven-baked chicken and ribs, ham, chitterlings, greens, candied yams, and a wide selection of desserts that are prepared in-house. They are located at 6495 New Hampshire Avenue, Hyattsville, MD, on the second floor of a bank building. Their telephone number is 301-599-4600, and they have unusual hours, so prospective visitors should call ahead to make sure that they’re open. The second is the Florida Avenue Grill, which is reviewed by the Sterns elsewhere on this site.