There's another thread which reviews restaurants but the reviews are conflated with fast-food, casual dining and ethnic restaurants. So, this one will be dedicated to ethnic food.
El Mariachi Taco Factory, located down the block from Plaza Foch and on the corner of Mariscal Foch and Juan León Mera.
This was the second time I went, the first for takeout, but this weekend dined-in with a friend and I liked it even more. The ambiance is tranquillo as the locals would say, and despite a loud party atmosphere outside on weekends this place isn't affected by the madness because it's located on the second floor. Their menu is just right with about a dozen main dishes. They start you off with nacho appetizers which are served with jalapenos and three dips, guacamole, salsa, and sour cream. The main dish was Fajitas which was an assortment of meat and chicken atop veggies on a hot plate. One plate is $11 but more than enough for two people, and with leftovers, yes, the portion was huge. This came with tortillas, and same condiments as appetizer. Very satisfied with the flavor, and the service was prompt. The total cost was $16, which also included two large pilsner.
Overall, definitely will go there again because it's centrally located, nice ambiance and of course delicious reasonably priced food.