The "Best" Lands?

 Here's a map that talks about the overall soil performance in the world. The arability of land is by far the most important in growing and maintaining a pre-Industrial population. You need to feed people, and without transport and refrigeration, most of that foodstuff is going to be local.

What's very interesting is the gradient that follows the Middle Ages, and America in the new Middle Ages, is that it's not the best soils to grow crops aren't necessarily the best to grow people. As White has mentioned, Ukraine, the Great Plains, Argentina are breadbaskets in the modern world. It's the areas that have slightly less inky, one might say (fittingly enough) an "army green" that have the densest populations, even those soils tend to be a little fractured. China, India, France, the Mediterranean, and in America, the Great Lakes/Ohio Valley region. (Also, Ethiopia, which was considered the place in Africa to have a social structure similar to Eurasia)

So the best places to look for places in the Post-Colonial (mainly Southern Hemisphere) with the the largest populations most organized, recognizably medieval societies would be in Venezuela (seeping into Colombia), South Africa, Southwestern Australia, Northeast Brazil as well as the Parana River area. Mexico would be particularly interesting as a study as it most ideal soils are actually around the coasts, whereas Mexico is somewhat notorious for having the majority of its population (and the center of the Aztec Empire) in the more inland highlands.