The large concentration of Latin American immigrants makes for stiff competition in San Francisco when it comes to tamales. There are tamale ladies with Twitter handles that deliver by bicycle, there are many restaurants in the Mission District that offer many tasty varieties as well. But when it comes to food trucks, I think the best is Alicia's.
She can be spotted traveling the normal truck routes including SoMa StrEat Food Park. The offerings include pork, chicken and veggie and aqua fresca as well.