Finding the best tacos al pastor in Mexico City is akin to finding the best baguette in Paris. There are so many places of exceptional quality; the differences come down to stylistic nuances in flavor and preparation. The best taco al pastor in Mexico City is the closest taquería nearby above a minimum quality threshold that's open late-night. Fortunately, the city is full of such restaurants. One of them is El Charco de las Ranas, or "frog's puddle." It's a sit-down restaurant as opposed to a street-side taco stand. The authenticity police might get angry that real tacos are to be consumed sitting down, but rest assured this is the real deal. There's a full kitchen in the back with a fire-roasted rotating pork spit by the door. The smell of roasting pork permeates the entire restaurant.
Service is quick enough that it doesn't make sense to order more than two tacos at once -- better to eat them immediately after they're made. It's essential to order at least a taco al pastor; after that order whatever sounds good.
Taco al Pastor - Tacos al pastor are abundant to Mexico City. Some taquerías serve al pastor tacos exclusively. In these tacos, shreds of pork and pineapple garnished with cilantro and raw onion. Cradling the meat and fresh vegetables is a glistening corn tortilla. The tortilla fries in the dripping pork fat below the spit lending to its glimmer. A skilled taquero is able to slice a wedge of pork and pineapple in one fair swoop directly into the amber-gold tortilla. The combination of sweet citrus with salty roasted pork and corn tortilla makes this, perhaps, the best tasting type of taco.
Taco de res - Chunks of fatty beef a top a corn tortilla. The taco is served quite plainly and garnished table-side with a variety of salsas. The guacamole is pure, creamy, and thin; chunky guacamole seems to be a state-side invention. I put on a dab of pico de gallo, but I seemed to be the only person at the table doing that.
Red pozole - Pork stew with giant kernels of maize.
El Menu - Lots of choices but the menu centers around the tacos.
Tacos al pastor are considered fast-food and the taquerías that serve them are usually open quite late. This makes tacos a popular late-night snack, particularly in the early hours of the morning after going out. They can be quite greasy as the spongy tortilla absorbs most of the pork fat when it's fried. Tacos al pastor are one of the dishes I crave the most when I am outside of Mexico.
When in Mexico City, I'd recommend finding a few of the nearest taquerías that serve tacos al pastor and trying them out. Hopefully, it's walking distance from El Charco de las Ranas.