 Is he the most famous among Americans? I don't know. But he's the most famous athlete in America. There's no doubt about it. His team, Miami, just with his presence, went from not making the playoffs to one of the best teams in the league just because 36-year-old Lionel Messi joined the side. Second most famous athlete in America. Who is it? Well, when you consider how long he's been retired, this really is impressive.