I was in Mexico City last weekend and didn't get much chowing in, but I did have delicious tacos from two places in particular.
On Isabel la Catolica between Mesones and Regina on the West Side of the street is a little taqueria the name of which I couldn't discern. They have a spit with a big heap of al pastor meat, complete with a pineapple on top. The tacos were about ten pesos (one dollar) each, and were fantastic; the pineapple and the pork complemented each other wonderfully.
Last Sunday I was strolling around and happened upon a stand in the street at the corner of Bolivar and Venustiano Carranza. I recognized the barbacoa (goat) taco smell from my Brooklyn neighborhood. I love barbacoa tacos, and the proprietor was cutting up the goat leg on the stand. I got one of the tacos -- again, ten pesos -- and it was, again, delicious, especially with the availability of everything you'd want to dress a taco with: lime, cilantro, onions, spicy sauce.