Best Tacos in SF--Tonayense Taco Truck
Believe it or not, the best tacos in the City can be found on a taco truck between 19th&20th on Harrison Street. These aren't your typical "Gringo-Tacos", rather they are more authentic Mexico City style tacos. Don't expect more than just great tortillas, greasy carne asada, cilantro, onions and hot sauce. Each will set you back $1.25, so order 3-4 if you're hungry--they aren't huge like what we are used to. Horchata, a cinnamon/rice drink, is an excellent choice to wash them down with. I'm not sure their hours, but they have a regular restaurant at 24th & Shotwell where the tacos are still very good, but not as great as off the truck.
Welcome to Chowhound, Blaine. You'll find many fans of taco trucks here, especially of Tonayense. The al pastor is my personal favorite from Tonayense.
Tonayense also has a truck on Shotwell just south of 16th Street, which is an excellent place to fortify yourself when you're getting your car washed around the corner. Order your taco "con todo" and they give you a piece of lime and one of the pickled peppers from the big jar, which you don't get at the more gringo-fied places. They also have excellent red salsa (which I know from the place on 24th), but they don't put it on the al pastor tacos.