The best taco truck around, by far, for the past several years has established a permanent location next to Arlington Cinema and Drafthouse on Columbia Pike. They were located in the parking lot of the Food Star, but now that the whole strip center is being renovated, they decided to go another route.
This is one of the few trucks that did everyhting well, so I am not sure I can give you a specific rec. Everythign here is cheap enough to warrant plenty of exploration, which I intend to do fully. If I find any favorites, I 'll post them here.
Until then, this is a place where you don't have to be shy. Order with confidence, and a couple of bucks toward a taco outside of your comfort zone may surprise you.