Cecilia's
Cecilia's, 2619 Columbia Pike, Arlington, VA, 703-685-0790, usually open (Metro Trip Planner - opens in new window) [WaPo | Yelp]
The main Bolivian place in the area, and a center for Bolivian social life. Excellent food, I really like the Pork Fricasee. The menu is not purely Bolivian (Mexican specialties are offered), but you should probably order Bolivian, even though everything looks good. Music and dancing at night on weekends.
The key to Andean restaurants in the U.S. is to avoid ordering a plain piece of meat; the quality of the meat will not be bad, but it will not be high enough to justify your choice. Look instead for sauces, stews, soups, and concoctions. Or a piece of meat with lots piled on top of it, especially chiles and green sauce.