I had the meal I remember with the most fondness on my whole trip to SF here at this plain storefront of a taqueria in the Mission District. My friend and I were walking through the district near his home and wandered in here via serendipity; we were planning to eat in Chinatown, but the restaurants here looked promisingly "authentic".
For a New Yorker, Cal-Mex is a revelation; it's a cuisine, not an excuse for margaritas. El Tonayense is a storefront with ugly orange plastic booths. There's a list of meats and fillings possible rather than a menu - you can have them in a burrito or a taco. I chose the burrito al pastor and my friend chose the carne asada. Each was a little more than $5. After you place your order, you're given tortilla chips and there's a self-serve sauce and salsa bar. The burrito was delicious. "Al pastor" (shepherd's style, I guess) is with barbecued pork; like any secular Jew, I love the stuff. The burrito was brought to us in a plastic basket, a foil-wrapped torpedo. The meat was delectable and the other fillings -- guacamole, sour cream, and vegetables -- fresh and lively. It was a filling and delicious lunch and perhaps the biggest delight because it was so unexpected.
There are several other taquerias on the same block; I'd guess that many of them are good, but I found out later that El Tonayense is well known and has taco trucks as well a few blocks away. They're even more renowned for their tacos than their burritos. I wouldn't travel hours for a burrito in a fast food joint, but I'd make a detour to eat there!