If you are in North OC or Lower East LA
Just stumbled across this site, but I had to mention my absolute favorite place of all time, Puffy Taco. It's in Whittier (or La Mirada, I'm not sure which), off Leffingwell and Santa Gertrudes. I have been living in Philly (which has no decent Mexican fare to speak of, but that's for another forum) for 5 years now, but I come back just for the food here (and maybe visit relatives). It's pretty much a stand, but there's also seating inside. I always get the enchilada plate, but their specialties are puffy (fried) tacos, enchiladas, carne asada, refried beans..., well, just about everything.
They have a second place in San Antonio. I drove around looking for it, but I never found it. Anyway, the people are very cool, and the food is cheap and very, very good (unless you're on a diet or something).