Which civilization built the city of Teotihuacan's great pyramids' region later found by the Aztecs?

The Teotihuacanos, and we do not know what they called themselves. Teotihuacan is an Aztec name - the place where the gods were born - given by people who arrived centuries after the city had been abandoned, found ruins on a scale nothing in their world matched, and concluded it must have been built by gods rather than by anyone.

At its height around AD 400 it held perhaps 125,000 to 200,000 people, which made it one of the largest cities on Earth, laid out on a rigid grid with multi-family apartment compounds - not palaces and huts but planned housing, which is unusual anywhere in the ancient world. It had no obvious dynastic art: no king lists, no ruler portraits, no royal tombs identified with certainty, which has led to serious proposals that it was governed collectively rather than by a monarch.

It burned around AD 550, and the burning was concentrated on the civic and elite structures along the main avenue rather than being general - which points to internal revolt rather than invasion. Its language, its writing, its rulers and its name are all lost, and it is the largest pre-Columbian city in the Americas about which almost nothing political is known.