The United States. More tornadoes occur in the U.S. than anywhere else on Earth. Why? It has to do with our continent's unique geography.
First, we have not one but two sources of hot, humid air: The Altantic Ocean and the Gulf of Mexico. Second, we have an extensive, flat plain that stretches all the way to Canada, providing a pathway for cold, dry air from the north. It's this clash between the two different types of air that forms the basic building block for tornado-producing thunderstorms. Third, we have the Rocky Mountains to the west which cause air flowing from the Pacific Ocean to sink, dry out and warm, forming a large inversion (a layer of warm air above the surface to the east). Initially, inversions hold back cloud development, but eventually, they erode, allowing large cumulus clouds to form. And finally, we have a desert in the southwest that produces dry, arid air which also clashes with the warm, moist air from the Gulf and the Atlantic.
Nowhere else on Earth do you find this combination of factors working together to maximize storm potential.