though palo alto sucks for mexican food, the best places in SF are far, far above and beyond anything in boston -- anna's doesn't even care to compete. try el farolito or el castillito. my favorite in the boston area is tacos lupita, in somerville (it's where the people who work at anna's eat.)
Eww.. read Farolito's health score card and see if you still want to go back there.
Walk two blocks west to Papalote's - 24th and Valencia. IMHO, best (cheap) burritos in SF.