International Blvd is great for the variety, volume, quality and ability to check out a lot within a short range but the OP mentioned SF.
I second El Tonayense. It gets mentioned for a reason. I like the torta al pastor (sandwich).
Also there's Los Compadres at Spear and Folsom, closer to downtown in SOMA. I've been a couple of times and it's as good as any truck I've been to, although I don't know the consensus best item...the carnitas is v.good.
Los Compadres Taco Truck
292 Spear St, San Francisco, CA
Bypass SF and go through Oakland. The best tacos are to be found near the Fruitvale BART station, mostly along International Blvd. There are 5 or 6 trucks and I don't recall all the names, but they are all good. In particular, don't miss the El Ojo de Agua truck on Fruitvale St, a block west of International. It's just one man's opinion, but I haven't found a single taco truck in SF that compares.