Trying to say who was the all time best in any particular sports is tricky territory, not to mention subjective. But here's a few thoughts.....
The NBA. I would suggest Julius Irving. Dr. J didn't win a bunch of titles and wasn't the best in any particular facet of the game. But he revolutionized it. Yes, there's Michael, Lebron, Magic, Bird, and many others over the years that have captured our attention, but all the "did you see that?" plays had their origins with Irving.
Major League baseball. The Hall of Fame has some mighty notable names in it for various reasons, and no game features more stats from hell these days than baseball. But give me George Herman Ruth. No one player was ever bigger in his sport than the Babe, who remains a legend to this day -- almost a century after he began wowing the world with his formidable bat.
The NHL. Once upon a time, it was Gordie Howe. But then the Amazing Gretzky came along to break all his records -- by a lot. Though occasionally a hot-shot prospect (see Mario Lemieux, Sidney Crosby) is referred to as the "next Gretzky", there will likely never be another quite like Wayne.
The NFL. The trickiest of them all. Different players have different specialties. Statistically speaking, OJ Simpson was far and away the best running back. He averaged over 12 more yards a game during his long career than anybody else. That's a bunch. Remember, the NFL only played a 14 game season in OJ's days. Jerry Rice has to be right up there with the best receivers. Lawrence Taylor likely combined speed and strength to be the most feared defender. But the best of them all might still be playing. Enter Tom Brady, once only a lowly 6th round draft pick. Forget Deflategate, nobody has done more with less than Brady and head coach Bill Belichick. Every year they lose star players but the beat goes on in New England -- as in winning Super Bowls.
The PGA. A few years ago, this appeared to be a no-brainer. Of course it was Tiger Woods. From 1998 to 2008, he turned in arguably the most dominant decade in ANY sport. Eldrick was seemingly winning everything and a "lock" to surpass Jack Nicklaus's record of "major" victories. People were regularly taking Tiger over the entire field in any given tournament. That's was incredible testimony to his greatness. But then it all fell apart -- and rather quickly. In recent years, the young guns on tour have been beating Eldrick brains out, and Tiger's lucky to make a cut -- let alone win. So the nod for the greatest still goes to Jack. Tiger had 10 great years. Nicklaus had 25 really good ones.
Race horses. This is a slam dunk. It was Big Red, the late Secretariat. That colt was to thoroughbred racing what the Beatles were to the rock and roll scene. There will never be another quite like either.
Pro tennis. On the ladies' side, Billie Jean had her day but was eclipsed by Chris. She was in turn outdone by Martina, and Serena has ruled for the last few years. From a male perspective, Rod Laver was once the gold standard, but few would doubt many others have come along since that would beat him to a pulp -- even in his prime. Connors, McEnroe, and certainly Borg, who took an early retirement. Pete Sampras was the Tiger Woods of his era just a generation ago. Though aging, Roger Federer remains a top seed in any tournament he enters. Rafael Nadal once owned clay court tournaments, but has been hobbled with injuries in recent times. But like Tom Brady, the best of all time is likely still playing. A Serb. Novac Djokovic. He has it all. Big serve, power from both the forehand and backhand, drop shots, lobs, return of serve winners, pretty much every shot in the book. Plus incredible stamina. And he's been winning everything lately too. Factor in he's only 28 years old, and who knows what he can achieve in the next few years? If he stays healthy and motivated, neither a given, he might well turn out to be the best of all time in tennis.
Boxing. Many say Muhammed Ali was the greatest of all time. Yours truly has long disagreed with that. True, he was a heavyweight and the big boys typically get more attention than most of the lower weight classes. But Ali got beat five times. Joe Frazier rocked him in their first fight. Ken Norton broke his jaw and thoroughly punished him in a non-title bout. (Idle thought. If one is the champion, how can they fight without their title at stake?) Later he was humbled by Leon Spinks. Leon Spinks!! Still later, he would get in the ring with a much better version of his old self, one Larry Holmes. That was a huge mistake. Holmes destroyed Ali. Sure, Ali was aging but, c'mon, he fought a lot of bums too on his way up the ladder. In a pitiful Joe Louis style swan song, Ali was even thumped by Trevor Burbick. Five losses should hardly add up to the greatest of all time. Floyd Mayweather is 50-0. Yes, he's easy to dislike, but 50-0 is 50-0, and he's fought all the contenders in recent years. But despite his bank account, Floyd was a lightweight in a few respects. Heavyweight champ Rocky Marciano retired undefeated as well at 49-0. A computer simulation several years back predicted a bloodied Marciano would have TKOed Ali in the later rounds if both had been in their prime for a fight. Oh my, the howls that generated. Good luck finding that tape. As for the greatest of all time? Give me George Foreman. Yeah, he got rope-a-doped in the "rumble in the jungle" a few decades back by the same Ali, but anybody that can make countless millions selling a handy-dandy grill with their name on it that they never invented in the first place gets my vote. Yep, I've got one too. Works great.
But the really, really, REALLY all-time greatest in sports award is reserved for non-athletes. These would be the people in the front office and marketing department of the Detroit Lions. They've had a pitiful team for over half a century, yet continue to raise their ticket prices, and the dang Motown fools keep scarfing them up like Elvis is back in town for a concert.