I recommend the Carnitas en su Jugo - it's a delicious plate of carnitas with onions and cabbage, and you can make your own tacos! It's to die for - and diet friendly, as pork is a white meat, and they have corn tortillas ;)
Tacos are $13 pesos and the plates are $65, I believe - trust me - the pictures barely do it justice - it's a LOT of food!
Stay tuned for more from this little known restaurant right in the heart of the city...