Greatest athlete of all time (US)

Gretzky may be my #1.  His accomplishments compared to his peers were just incredible, and some of these stats are ridiculous:

https://www.si.com/hockey/news/10-hilariously-ridiculous-stats-to-illustrate-wayne-gretzkys-dominance#:~:text=Gretzky%20won%20the%20scoring%20title,31%20points%20in%201988%2D89.

Just to cherry pick one example, "He could go more than 16 years without a point and still average a point per game in his career."

Jim Thorpe.  To compete at the highest level in the Decathlon and the Pentathlon in the same Olympics is the most remarkable achievement in sports history.  No matter the era.  He went against the greatest athletes of his time and won the two most difficult events in the games.  

Here are some names that I think you could argue should be in the Top 5: Jim Thorpe, Carl Lewis, and Jackie Joyner-Kersee.  

Can't believe no one is mentioning Brady.

Hardly the most imposing physical specimen, either now at age 44 or when he was in his physical prime (whenever that was). Wasn't the tallest, the biggest, and sure as hell wasn't the fastest, but that sure hasn't mattered over the last 20 years. Everything he brings to the table, from a mental standpoint, the clutch standpoint, having that "it" factor that just wills you to victory, you can't replicate that. There's nothing you can throw at him that he hasn't seen before. There is nothing that he doesn't know how to adjust to, find the weakness in, and beat. Seven Super Bowl rings, and he's got the potential to go get an eighth before this is all done. No other active player in the game is close to achieving what he has. 

If you measure an athlete strictly by their physical attributes, Brady isn't anywhere close to the top of the list. But when you take the whole package, to me he is right up there with Michael Jordan, Simone Biles, Babe Ruth, and Jesse Owens. All of them are great for different reasons, but they were all great athletes. 

Another all-time great that no one is mentioning: Jackie Robinson. He and Jesse Owens were probably the most courageous US athletes who ever lived. One shattered the color barrier in baseball at great personal risk to his own life, and another basically went right into Hitler's backyard and destroyed any and every ounce of BS there ever was about "Aryan superiority." Owens shattered that myth right in front of Hitler and the German citizenry. I can't imagine Nazi High Command was too pleased with the results of the games that year. 

Bo Jackson, Deion Sanders and Tom Brady should be in the top ten. 
 

You make the Secretariat argument, which I think is absolutely nuts. He's a horse. He can run fast. That is all. He lacks every other skill.

My argument would be for looking at the ability to use athleticism across sports, giving more leverage to athletes like Dave Winfield, Bo Jackson, Prime Time, Herschel Walker, etc.

This list doesn't surprise me but it's IMMENSELY mainstream male sport heavy.  Like, is Rodger Federer a better athlete than Gretzky?  I'd argue he is.  Serena Williams dominated her highly-competitive sport for a long time.  Simone Biles has shifted what's possible in gymnastics to a degree we don't probably comprehend as a society.  Alex Honnold's free soloing of El Capitan is truly one of the great physical achievements in modern history.  Brock Lesnar is an insanely-gifted physical marvel who was a world-class amateur wrestler, WWE performer, and MMA champion.   And as others have noted, Secretariat was truly a dominant force in horse racing.  The list goes on and on, and while I think all of the athletes listed here are worthy it's such a myopic outlook on what's considered athletic achievement.

Man O War before Secretariat.

Of the humans;

Jordan

Brown/Jackson I can't pick between them.

Ali

Howe

Ruth

I see it differently.... Gretzky was protected by the refs (though Jordan was too).

I place Bo Jackson and maybe Barry on that list.  

Secretariat was the only S.I. athlete of year despite being a horse.  He should be left off the list.

Bartolo Colon

Carl Lewis

Let's say it's appropriate to put a horse on the list for just a moment...I would still disagree because a horse generally only has 1-2 seasons of being an "athlete" and I think that longevity should play a role in how accomplished an athlete truly is.  1-2 seasons is considered a flash in the pan so I don't think it's really fair to include anyone on a list for such a brief period of excellence.  

I'm also a bit surprised that I didn't see Jackie Joyner-Kersee on most people's lists.  She was pretty spectacular in the Heptathalon over many years which essentially proves she was a GREAT athlete.

Way too many good candidates and variables to ever judge definitively but here are a few that came to mind.  Not necessarily in order.

Most physically gifted:  Bo Jackson, Allen Iverson, David Robinson, Lawrence Taylor, Roy Jones Jr, Champ Bailey, LBJ

Most accomplished athletic career:  Brady, MJ, LBJ, Jerry Rice, Serena, Martina, Carl Lewis, Michael Phelps, Barry Bonds*

BO JACKSON

Justin Feagin

Justin Feagin

RONNIE COLEMAN

I would go with:

1 Bill Russel               Basketball               
2 Ty Cobb                  Baseball         
3 Muhammad Ali        Boxing
4 Tom Brady               American Football
5 Gordie Howe           Ice hockey

1) Dick Weber

2) Pete Weber

3) Earl Anthony 

Bo Jackson, Deion Sanders, Jackie Robinson, Adrian Peterson and Jim Brown. Those are all “athletes” and not just stuck to one sport.

Best athletes....bo Jackson and deion sanders because they were good at multiple sports, freak athletes. I know Deion couldn't tackle but he is probably the best cornerback ever. Bo is Bo. In a few years you may be able to say Ohtani for what he is doing and if he stays healthy.

My third is Simone Biles. She is so dominant and changing her sport because she is doing stuff not a single other woman on the planet can do.

A horse is horse, of course of course. But I've seen no one like Secretariat as a force...