First let me start out by saying: Tacos, breh
I eat tacos damn near every day for lunch...
Here are my favorite spots:
Zona Rosa on Haight St. in SF
Papalote on Fulton in SF (I know it isn't a taco, but they also have this concoction called a chicken adobo burrito...best burrito I've ever eaten) this is also the same place that won a burrito throwdown against Bobby Flay.
Ruddy's Taco Truck ($1 tacos all day breh) usually posted in San Bruno near Tanforan
In LA...I always have to stop by King Taco in East Los...I love that damn place...hottest hot sauce ever. Plus, the security guard carries a revolver on his hip, so you know the tacos must be good.