And if you are in Salt Lake City, don't miss The Red Iguana. Not really a taco place, but serves the best mole outside of Oaxaca I have ever eaten. They serve green, red, yellow, brown and black moles, all with distinct and succinct flavors.
And if and when you are in Mexico City, try these:
El Izote, Presidente Masaryk 315. The peak of Mexican food. The lobster enchiladas are treasonable, and the steak with cream, huitlacoche and rajas is beyond belief.
La Valentina, Presidente Masaryk 393. Traditional Mexican regional specialties, with the best sangritas - tequila served neat in a small, narrow glass with a side of tomato-based chaser.
Cafe de Tacuba, Tacuba #28. Housed in a an early colonial villa, in the heart of downtown. this place serves great caldos and sopas and deserts, features fine live music that is not just mariachi, has great aguas frescas and beatiful rooms. A real family place.
Sanborn's at the Casa de los Azulejos. Go here for great Mexican breakfasts. The house is tiles outside in blue and white, and the airy, three storey interior has skylights and Orozco murals on the stairwells.
And in Coyoacán, a suburb inside of Mexico City (!) there is a fabulous oaxacan restaurant called Los Danzantes, Plaza jardín Centenário #12, www.losdanzantes.com.mx.
You sit on the plaza of Coyoacán, across from the house that Hernán Cotrez lived in, adjacent to the beautiful cathedral, near the Frida Kahlo/Diego Rivera house and museum, near the house that Leon Trotsky lived and was murdered in, and next door to many neverias, sherbet houses, where you can eat great flavors like tamarind, guanabana and guayaba. Los Danzantes is simply superb. They have authentic dishes like Hoja Santa - holy leaf - which is just a big thick leaf with sauce on it, and then it tastes like asparagus, mint, lemon-grass and you just go wow. They also serve mezcal there distilled in their own agave hacienda.