In the US the Midwest region produces the best US fighters... its more about quality than quantity though.
Mexico City has produced many quality fighters as far as cities outside the US go....As for the US i'll have to go with Philly.
Detroit/Flint has produced a ton of great fighters: Thomas Hearns Sugar Ray Robinson Joe Louis Andre Dirrel Chris Byrd James Toney Milton McCory
Detroit, Philly and New York seem to stand out the most IMO. BTW how could you leave out Detroit on the poll?