My favorite sandwich (ever) is the Hawaiiana, at “Tortas Chapultepec,” turn left out of the front of Hotel Camino Real in Polanco, and it is on the corner at Victor Hugo and Mariano Escobedo. They usually are open by 9:30 and I suspect they close fairly early.
Pujol does wonderful things with vegetables and is perhaps the best fancy place to try; I recommend the Menu de la Tierra.
They have done away with the food stalls at the Zócalo. In Mexico City calorie-counting menus are common and gelato is being replaced by frozen yogurt (!).
Tres Marias is a “food village” right off the highway on the way to Cuernavaca. Look for the place on the southbound side which specializes in green chilaquiles and also chorizo tacos, but in general standards along that strip are remarkably high.
Overall, Mexico City is becoming a safer city, and compared to four years ago one sees many signs of economic progress.