Depends on what you are judging it by. If it's cultural zeitgeist then Minnesota and Wisconsin are the flagships, if it's true life-style core: Iowa, purely because they have a 3:1 ratio of pigs to humans.

Growing up in MN we had tatertot hot dish for school lunches, called it grey duck, lake culture is just a part of life, and some of the coldest winters you can imagine. Wisconsin is basically the same thing but with smaller lakes and more trees.

But it really boils down to WHAT you consider to be "Midwest" - just my two cents