I am from El Salvador, so I can assure to you that at Ercilia's you can have one of the best pupusas in Washington D.C.; I tried it for the first time more than a year and half ago, and it is my favorite place to buy pupusas from. The place it is very authentic, since the moment you enter you will be transported to El Salvador. It is not fancy or charming, but the food is very good, and the pupusas cost around $2. You should try also their "frescos" of horchata and marañon. My favorite pupusa is cheese with loroco. Also, it is very easy to access by public transportation (buses and Columbia heights metro station). The waiting time can be a little bit long sometimes but it is worth it.