Recently, I've made an effort to sample them more, since I work downtown. Many of the trucks cluster around Metro Center, which isn't too far a walk for me to grab something that's satisfying and under $10 for lunch.
Three trucks so far have stood out.
Takorean. As the name implies, Takorean is a Latin-Asian fusion of Mexican and Korean flavors. Hence, the beef bulgogi tacos, corn tortillas topped with Korean grilled beef, kimchi slaw, cilantro and lime crema. It's a fantastic idea, which I've indulged in twice and loved both times.
Sauca. Probably the most eclectic food truck menu, as Sauce prides itself on a menu with tastes from all over the world. Recently, I enjoyed their pork bahn mi, a Vietnamese-style sandwich which they serve in chewy flatbread with spicy-sweet shredded pork, pickled slaw and peanut-coconut sauce. It's almost too sweet, but nonetheless quite delicious. I've previously tried their Mumbai butter chicken, which was good too, with roasted cashews.
TastyKabob. Gyros are a food truck staple and this is a good place to get them when you're in the mood for a piece of soft, chewy pita piled high with lamb chunks and vegetables. I was recently irritated when the truck wasn't where their tweet said it would be (I wandered all over Judiciary Square twice trying to find it). However, my ire was squelched by the gyro's tangy dill sauce on a recent visit.
Many trucks tweet their location, so if you have a favorite, you can follow it. To see lots of food truck options, Food Truck Fiesta aggregates and maps these location tweets, allowing you to see an updated view of where trucks intend to park for the day. The site also has news about trucks, such as information about new ones. A recent post about an alleged police crackdown shows that the role of these trucks in the DC food chain is still a bit uncertain. The city is expected to clarify this with new regulations later this month.