As I mentioned below: Toscana and Panoramica Super Cocina are terrific. Let me now add Biche Pobre on Calz. de la Republica, just east of the Parque Juarez (Llano). They have 12 very Oaxacan appetizers on the menu, and for 65 pesos they serve a selection of every one of them. The order for one is enough for two average appetites.
Also Los V Hermanos several blocks east of the zocalo on Hildago has excellent pastries and quiche. The cream puff filled with chocolate mousse is 8 pesos. It has three tables, seats nine, and closes at 8:30.
The pumpkin flower soup at Catedral is a wonder, and their breakfast is good, too. Its on G. Vigil one block north of the Alameda.