The Sporting News conducted a bracket-style poll that found Detroit to be the best sports city in North America (it beat out Chicago in the final and Montreal and New York in earlier rounds).
I'm biased, but I think it's fair. Detroit is one of few cities that has 4 major sports teams that are all pretty much equally relevant, and 3 of them play downtown. Also, the Michigan-Michigan State rivalry lives within metro Detroit, as people there move west on weekends to cheer for their teams. As a city, fandom is balanced but still fanatic and passionate about their teams.