Michael Jordan? Wayne Gretzky? Deion Sanders? Anderson Silva? Mike Tyson? Bo Jackson? Muhummad Ali? Pele? Each dominated their sport but were they really once in a lifetime?
As far as I can tell, only Usain Bolt, Tiger Woods, Lance Armstrong, and Alexander Karelin can legitimately lay claim to that title.
By the way, this is MMA related because we constantly hear this about Fedor, Anderson, GSP, and BJ Penn.
Jack Nicklaus > Tiger Woods
Most of these athletes will have their records or feats surpassed in our lifetimes
Even though I'm not a huge fan of him, I'd say BJ. So many factors shaped him perfectly for success in fighting. Natural flexibility to a crazy degree. Very wealthy family able to pay for the best training. One of the best chins around, and the reflexes and toughness built from lots of street fights. High aptitude for both boxing/kickboxing and BJJ, and blending them together, even when new to the sport.
While I believe there are better fighters, and fighters who have worked harder to achieve greatness than him, I doubt we'll see anyone quite like him anytime soon.
Also, Karelin. But BJ was my pick for someone in MMA
I think Zidane put himself up there with the all time greats in soccer
Messi.
will go down as the greatest.
as far as MMA, I'd say GSP.
I think Bo was once in a lifetime. We didn't even see his prime imo. 
HogWatcher - I think Bo was once in a lifetime. We didn't even see his prime imo.
This.
Karellin is the real answer.
Micheal phelps.
Mario Lemieux.
No one else will ever have that combination of size and talent. Not in my lifetime.
Kobe Bryant, Lance Armstrong, and Hacksaw Jim Duggan.
Karelin. Dude was a freak of nature.
Shane Warne motherfuckers!
Joey Chestnut
Kelly slater
Gretzky could have scored 0 goals in his career and still be the all-time NHL points leader. No one will ever touch his numbers.
Ansari - Gretzky could have scored 0 goals in his career and still be the all-time NHL points leader. No one will ever touch his numbers.
This is true.
Some of the records he set are damn near impossible for anyone to beat.
92 goals in a season? 50 is a big accomplishment nowadays and he nearly doubled that.
Secretariat
I think Lemiux is the only recent player who had a shot at some of Gretxky's records but unfortunately illnes and injuries derailed his chances.
Actuallly, I'm pretty sure he would have been the all-time goal scorer if he played a full career.