Right now, I know that the city of Rockwall, Texas, is the fastest growing in the U.S. Being on the outskirts of Dallas, it could grow to be very large. But that could change in the next 100 years!
Since we're talking about metro area, I vote Denver. Because in 100 years, I bet it'll be one giant metro area from Cheyenne down to Pueblo, picking up some towns from east and west along the way.
Right now, I know that the city of Rockwall, Texas, is the fastest growing in the U.S. Being on the outskirts of Dallas, it could grow to be very large. But that could change in the next 100 years!