Las Cuatros Milpas in San Diego. Meets all criteria for the ultimate in authentic hole-in-the-wall MX food:
1) Located in the (in)famous Barrio Logan district (100% Mexican community)
2) Line out the door, by MEXICANS.
3) Minimal menu with only the essentials (a couple of taco choices, rice/beans/chorizo, sopas, tamales ... and that's kind of it)
4) Taco shells are fried ON ORDER (not sitting under a warmer).
5) Food prepped by 4-5 old, MEXICAN grandmothers.
6) Full meal to be had for < $10 (maybe even less, can't remember prices, just that it was pretty cheap).
Edit: Read and weep on Yelp:
http://www.yelp.com/biz/las-cuatro-milpas-san-diego