The country of Mexico has as many culinary styles as Federal States, Regions, Cultures. The food diversity is so rich that you can spend a life trying them all.
I believe that there is no major concentration of fine Mexican Food Restaurants than in Mexico City.
I would recommend you to try Guadiana and order the fish carnitas or go to Los Canarios and order the duck tacos. If you want to try the most sophisticated dishes, then go to Pujol.
Enjoy and keep exploring...