I was flipping through my Goode's World Atlas and came upon a world climate map, which explains a lot about why the world works as it does.
What looks like the most desirable climate zone for humans is Humid Subtropical. If you drew a line from Washington to about Wichita and then down to Corpus Christi, south and east of that would be the Humid Subtropical zone in the US. Its characterics are good rainfall, warm summers, and cool winters. Other important stretches of Humid Subtropical land are China south of the Yellow River valley, most of Japan, South Korea, and the northern Indian plain, helping to explain why so many people live in those areas. And there's southern Brazil and the Rio de la Plata drainage basin, also well-populated but ridiculously poor compared to what it ought to be. The Europeans who immigrated en masse to that area between about 1880 and the First World War managed to set up just disastrous governments at about midcentury. They tried almost everything but capitalism and democracy, and nothing they tried worked. What they wound up with was a system reminiscent of "bossed" cities in America in the early and middle 20th centuries. Now Argentina is a basket case and Brazil will be pretty soon, and there's just no excuse for this. They can't blame the Americans for the way they themselves screwed up and keep screwing up. You could understand their being poor if they weren't so obviously living in a place with all the geographic advantages of the southeastern United States.
North of the Humid Subtropical zone in the US is the Warm Summer Humid Continental zone, which also looks to be a great place to live. Draw a line from Boston straight across to about Buffalo, Flint, Green Bay, Minneapolis, and Fargo, and then down to Wichita, and Warm Summer Humid Continental is inside it. The winters are colder than in Humid Subtropical, but the rest is pretty much the same. The only other large Warm Summer Humid Continental zone in the world is China north of the Yellow River valley.
North of the Boston-Fargo line is the Cool Summer Humid Continental zone, which is not such a nice place to live in many Americans' opinion. It includes the inhabited parts of Canada west to Sasketchewan. In Europe, Oslo, Stockholm, and Helsinki are in this zone, as is Poland and most of the inhabited parts of Russia. In this zone winters are very cold and summers cool. No wonder Russia has never been a prosperous country. You try and try and the climate's still like Duluth, Minnesota. Or worse, because a lot of Russians live in the next zone up, Subarctic. No one lives in the Subarctic part of Canada. It just isn't worth it for us to settle large communities of people up there. But the Russians didn't have much other choice except to expand into Siberia--and then Stalin made them go out there and build cities.
The other important zones in the US are Mediterranean, which includes Southern California, most of the Mediterranean coast including nearly the whole of Iberia, parts of Iran and Turkey, part of Australia, Capetown, and central Chile. This is a fairly small zone in terms of land area but a lot of people live in it. The rest of the US Pacific coast is Marine West Coast, along with most of Western Europe, another small zone. A lot of the Southwest is Tropical Desert, and the Great Plains are Middle Latitude Steppe. Tropical Desert has never been desirable, hell, it wasn't even habitable before aqueducts and air-conditioning, and Middle Latitude Steppe is pretty marginal land, not worth much without irrigation. It can support a sparse population, which is what it has in America. I mean, nobody lives in Wyoming or North Dakota. Or western Kansas.
Really, if you look at it, two-thirds of the Earth is water and one-third of it is land. I'd bet that the sum of the four most desirable climates, the Humid Subtropical, Warm Summer Humid Continental, Marine West Coast, and Mediterranean zones, isn't more than one-fifth of the land on Earth. (So one-fifth of two-thirds is two-fifteenths, which is the amount of land on Earth that is really desirable.) I'd also bet that two-thirds of the people on Earth live in one of these four zones, and I'm pretty sure that these are the only four zones either you or I would want to live in. Exception: what is called Undifferentiated Highlands on the map, which includes areas where the different climates depend on the altitude. Denver and Mexico City are both among the Undifferentiated Highlands.