Peurto Rican Restaurants?
Rumberos is alright, though their mofongo is nothing to call your grandparents in San Juan about. The plantains were not as soft as I would have liked, and were presented as a basket for the shrimp ceviche-type filling that came in its own ramekin. Perhaps my mofongo preferences were too strictly formed by low-end restaurants that served grapefruit sized balls of crispy, fried plaintain with meaty fillings, but Rumberos' mofongo feels inauthentic. They are a decent restaurant, but not something I would travel for (my mother's recipes serve me perfectly well when I'm in a Puerto Rican fix).
Apparently Rumberos in Columbia Heights has mofongo. I don't know if it's more Puerto Rican or Dominican, nor if it's good. I believe it's owned by the same people as Rumba in Adams Morgan, which I think has pretty good food. But both are "pan-Latin" and so not really what you're asking for. The Islander on U St. is "Caribbean" -- seems more Jamaican, and I haven't been there either.