Form #5 includes the
top 10 markets of cities that already have teams (for fictional leagues or in case someone someone wants to give that city another MLB team):
New York, New York
Los Angeles, California
Chicago, Illinois
Philadelphia, PA
Dallas, Texas
Atlanta, Georgia
Houston, Texas
Washington, DC
Boston, Mass
San Francisco, California
EDIT: I'm not including Toronto yet as the Nielsen ratings don't include them. Nor am I yet including cities like Anaheim that might be included in larger markets.