Taco Loco is on its third South Side location (now at 27th & Jane) and is still serving up authentic tacos & mexican plates for a reasonable price.
The tacos are small, but great: adobo, chorizo, lengua, pollo, barbacoa, and more. Try the pambazo (which must be Mexican for Primanti Brothers); it's a tasty grilled sandwich stuffed with potatos & chorizo. Soyrizo is a fine available alternative for vegetarians.
As it does not yet have its own liquor license, you'll have to bring a few bottles of Negra Modelo yourself.