John Elway, formerly with the Denver Broncos, would be the most currently popular, although he is not homegrown. Jack Dempsey (1895-1983) , from Manassa, CO, was considered one of the worlds greatest heavyweight boxers.
Utah is not popular in sports... John Stockton.
Steve Prefontaine
Troy Aikman
Brett Favre
Darryl Sittler
Michael Vick
Perhaps the best known Iowan in sports history was Bob Feller.
georges st. Pierre
John Madden does he count?
Brian Urlacher
I would have to say Micheal Jordan.
Back in the day: Hakeem Olajuwon.