Cathedral Metropolitana
Sitting on the Zocalo in the heart of the city, Metropolitan Cathedral was started in 1573 and sits on the main site of Tenochtitlan- the heart of the Aztec empire.  It's no mistake that it's built on top of the old Aztec city- early Spanish attempts were to replace pagan monuments with Christian ones, and in the process, unfortunately wiped out countless historical and cultural artifacts.  The Baroque cathedral was built to resemble the cathedrals in Toledo and Granada in Spain.  The inside, unfortunately, is covered in scaffolding while engineers try to save the building from uneven sinking into the soft former-lakebed that is much of Mexico city.