So saith the President in his IOC pitch (whether that's something you want to brag about to the IOC I have no idea). He's a homer, obviously, but just doing some back-of-the-envelope thinking, maybe he's got a point:
* It's been an industrial, agricultural, and trade center over the course of the 20th century, and even as it shifts towards the last of those, there's still an influence of the first two.
* It came of age as a frontier town.
* I daresay it has a greater Southern influence than the nation's non-Southern metropolises - DC, maybe - and you can't really have America without the South. Most prominently black southerners, but Uptown obviously had a nontrivial white southern and Appalachian population, not to mention a small but not insignificant Native American population.
* It's an exceptionally diverse immigrant city, and also very segregated - which isn't a point of pride, but it's pretty American.
* It's obviously the most Midwestern of American cities, and I'm not aware of another metropolis with a large Midwestern influence (Denver?). Maybe Los Angeles with the Okie influence.
* In my experience, Chicago's suburbs are very suburban, like Platonic ideal suburban, not to mention diverse in the spectrum of American suburbs (think Oak Park vs. Cicero vs. Hoffman Estates vs. Hinsdale). That may be the bias of exposure, but insofar as one can evaluate the suburbanity of suburbs, I think we make a strong case at minimum.
* The biggest knock against Chicago as the Most American City is, I think, our lack of influence as a cultural arbiter (which may go a long ways towards explaining the Olympics obsession). Los Angeles is the center of the movie and TV industry and is strong in music as well, while NYC is powerful in TV, publishing, radio, and music. Given the importance of entertainment as a cultural and economic export, you can't really discount that.
* Not to mention that the financial industry is centered in New York, and money is culture.
So... I dunno.