+1 on El Cholo.
Also, a fantastic walk up stand is Yucca's
in the Los Feliz area.
Seriously, the most authentic-amazing-mouth-watering-street-tacos you'll ever have.
There are now two locations: original location at 2056 Hillhurst is basically a wooden shack on the edge of a liquor store parking lot and the second, newer location is at 4666 Hollywood Blvd and has a larger seating area.