Morton's of Chicago - Generally known for their steak, they also make a mean burger. And the atmosphere is pretty nice for a burger joint - dark wood and brass, white linens. It is definitely on the pricey side, but a very tasty burger. Two locations in DC - Downtown at 1050 Connecticut Avenue, NW (202) 955-5997 or in Georgetown

Brickskellar - This place is better known for its outrageous collection of beers but it also makes a mighty fine burger - called the BrickBurger. Or if you are feeling a little exotic, try the Buffalo Burger. You can also play games upstairs - board games and organized card games. A fun place. Inexpensive. 1523 22nd Street, NW, (202) 293-1885

Mackey's - Seriously huge burger with greasy fries in an authentic Irish pub. Sit in a snug and sip some Guinness Stout with your meal. Moderately priced. 1823 L Street, NW, (202)331-7667

Johnny Rockets - the motif is a 50's diner and the tableside jukebox costs a nickel, but the first song is on the house. The burgers are smaller than you might find elsewhere but so fresh and delicious. Crispy fries are great, and you can get them with chili and cheese. Don't forget about the chocolate malt. Inexpensive.
Two locations:
-Pentagon City Mall, Arlington, VA (703)415-3510
-4828 St. Elmo Avenue, Bethesda, MD (301)656-2050

Carlisle Grand - Along with the great salads, great soups, great chops, great desserts, they also serve a great burger - but beware, no fries available!!! They do have delicious fresh cut potato chips which can be a pretty decent substitute.. There is an upstairs (more formal, more expensive) and downstairs. I prefer downstairs. Moderate (downstairs) to expensive (upstairs). 4000 S 28th Street, Arlington, VA (703)931-0777