Aztecs

I have heard that the Aztec capital--Tenochtitlan, located in present day Mexico City, more or less--was one of the world's biggest cities in 1500, just before the Spaniards arrived. According to the Wikipedia, "With an estimated population between 200,000 and 300,000, many [who?] scholars believe Tenochtitlan to have been among the largest cities in the world at that time. Compared to Europe, only Paris, Venice and Constantinople might have rivaled it."

Logic implies that the Aztec civilization, therefore, most have been one of the most complex an organized in the world. After all, it take a lot of food to support that many people, which requires some form of infrastructure and administration to ensure that the food made it from the countryside to the city. With no draft animals to pull plows or carts, (or ride around on), the number of humans who had to coordinate to grow and transport that much food was probably higher than in Europe. As for the city itself, most of the people there probably weren't priests or rulers, but artisans or administrators of some sort--implying a substantial (for the age) middle class of skilled people.

All of which implies a pretty darn complex civilization.

About which I know virtually nothing.

Compare to ancient Rome: I learned Latin in highschool, can name most of the major Roman deities off the top of my head, and can sketch a vague outline of Roman history and culture.

At the very least, this implies that there's a lot of interesting stuff for me to learn.