this is our favorite restaurant..my husband and i use Rancho la Lena as our benchmark for mexican food. cant vouch for its "real-ness". its just darned good food, served by nice people.
Rancho La Leña
2367 San Diego Avenue
San Diego, CA 92110
they serve breakfast (order the chilaquiles), lunch and dinner (shrimp cocktail and the combo taco plate)
Depends upon what you consider authentic, where you are and what kind of food you want to eat.
Most authentic - Super Comdia on University around 37th, next door to an asian restaurant supply house
Pretty darn good - El Rodeo across the street from Hoover High
Really good fish tacos and other unusual burritos - El Zarape, end of Park Blvd. right before it dead-ends into Adams Ave
Good imitation of Mexican street food - Las Cuatros Milpas in the Barrio
Carnitas - Carnitas Uruapan in Lemon Grove or Tijuana, Por Venir in the South Bay, Porkyland with locations in the South Bay and La Jolla. El Cuervo in Hillcrest
Do a Ctrl-F search for North County recommendations. There are many and they all sound really good
You can find Mexican food on 3 out of 4 street corners in most of San Diego. Most of it is "decent" at best, some of it is really good, little of it is truly awful.