San Francisco taqueria located in the Mission District, on Mission Street near the corner of 19th street (across the street from the San Francisco version of that social club for the young moneyed set going by the name of The Beauty Bar (avoid it at all costs!)).
If you aren't familiar with the concept of the mission burrito, then you should come to Cancun to initiate yourself into this world. If you are familiar, then chances are you've already tried Cancun's culinary concoctions.
Menu options at Cancun include burritos, tacos, quesadillas, enchiladas, cerveza, and agua fresca. The popular choice is the burrito and for between 3 and 4 dollars you can order a basic burrito with meat (or vegetarian). Meat choices include chicken, pork, suesos (beef brains), and lengua (cow tongue). A great thing about a Cancun burrito is the fact that the basic burrito comes with huge slices of avocado (no watery guacamole here). Get mild salsa unless you can handle the caliente stuff, and most people get black beans instead of refried pintos. For a special treat order ceballitos (fried green onions), but you won't find them on the menu. On a sunny day, a veggie burrito and ceballitos are the perfect order for a walk to Mission Dolores Park because you can eat the onions on the way, and enjoy the burrito in a beautiful San Francisco park.
Since the closing of Maya Taqueria across from Katz Bagels on 16th street, Cancun has moved back to the top of the list as my favorite San Francisco taqueria. Other excellent taquerias include: Azteca (run by former owners of Maya Taqueria) on Church and 16th streets, where you can get the best tofu burrito (with ranchero salsa) you've ever tried (or get the tofu taco for better value if you're broke), El Toro Taqueria on Valencia Street and 18th, Poncho Villa on 16th near Mission Street, and the ever-popular El Farolito Taqueria located at the 24th Street BART (NE Corner) where the flicker flourescene gives this consumer a massive headache (a fact little-helped by the greasy food).