Peoria, Illinois has absolutely no smell at all.
However, Clinton, Iowa my former hometown is the worst smelling place in the world. Take a Purina hog rendering plant (they make dog food), add an ADM corn processing plant and top it off with some unknown places that pump straight gray smoke into the sky 24/7, and you've got smell.
After Field of Dreams came out, some witty Clintonite started selling t-shirts with a map of Iowa on them. The map had a star over Clinton's location, and "The Armpit of Heaven" underneath.