Muhammed Ali (boxe)
Jack Nicklaus (golf)
Pelé (football)
ayrton senna (F1)
Roger Federer (tennis)
Ussain Bolt (athletism)
Michael Jordan (basketball)
Michael Phelbs (swimming)
Tiger Woods (golf)
Carl Lewis (athletism)
Sir Bradman (cricket)
I'm sure I'm missing a whole lot... But I lol'd pretty hard at people including guys like Brett Favres or Lebron James in here.
Or I didn't get the question.
I mean Greatest athlete ever? In the world or just the US ? Caus' I don't know if you're aware of it but american football is kinda exclusive to the US... The most majority of the world just don't give a fuck about this sport. So picking an athlete from your Baseball or football league is nonsense for me.
Football (the real one, soccer for you) is the most popular game in the world, by far. Then I guess I'll pick Pelé.. But it's way to complicated to pick a winner with such different sports. Federer could be the one, as it's sooo difficult to stay on top of the ranking in tennis for long period of time, you have to be insanly consistant to win 15 majors... I mean the guy cannot loose a single match when playing a Grand Slam, no day off possible, one mistake you're out.
Phelbs, Jordan, Ali, Pelé and Federer would be in my top 5.
noooooooow if the question is "fittest athlete ever", I guess you have to look into football(soccer), or maybe Rugby.