Cancun
Cancun, 11260 Georgia Avenue, Wheaton, MD, 301-949-9838 (Metro Trip Planner - opens in new window) [Yelp]
Mostly a Mexican and El Salvadorean restaurant, but they serve some Puerto Rican specialties on weekends. Sleeveless shirts are not allowed, that kind of place. Alas, no mofongos, however, which means not enough garlic for my taste.
See also Banana Café, under Cuban, they have some Puerto Rican dishes.