As Serena Williams closes in on retirement, it’s time to take a look at how we measure and rank athletes in terms of greatness.

Championships? Longevity? Versatility? Dominance? Character? Impact on society?

Many if not most barometers of sports success are subjective. That’s why it’s impossible — but fun — to compare athletes of different sports, eras and genders.

In 1999, ESPN enlisted a panel of 48 nationally known sports observers to count down the 50 greatest North American athletes of the 19th and 20th centuries.

Five women made the list. There were no race car drivers, no soccer players (Pele is Brazilian) and no MMA fighters. There was a horse, though; Secretariat was 35th, between Lou Gehrig and Oscar Robertson.

Also in 1999, Williams captured the first of her 23 Grand Slam singles championships. At age 17, she beat another phenom, Martina Hingis, in the final of the U.S. Open.

Michael Jordan, Babe Ruth and Muhammad Ali, in that order, were the top three on the ESPN list. The highest-ranked woman was Babe Didrikson at No. 10. Martina Navratilova was 19th and the top tennis player.

If that list is done again, it’s easy to make a case for Williams as top 10. Some even say she should be No. 1.

“Serena Williams, in my opinion, is the all-time greatest athlete in the history of any sport, male or female,” NBC Sports reporter Julia Elbaba said recently.

Considering she’s a former professional tennis player, Elbaba might be biased.

But who isn’t when it comes to sports?

Here is just a partial list of the many athletes from around the world who have made their marks since Y2K that — depending on how you look at it — belong on that all-time top 50 (my guess is the next time someone tries to do this the list will be expanded to 100):

David Beckham, Simone Biles, Usain Bolt, Tom Brady, Kobe Bryant, Steph Curry, Novak Djokovic, Roger Federer, Mia Hamm, LeBron James, Katie Ledecky, Floyd Mayweather, Rafael Nadal, Khabib Nurmagomedov, Shaquille O’Neal, Michael Phelps, Annika Sorenstam, Abby Wambach.

Several of these superstars should be in the top 10 … and fans with allegiance toward a particular sport, team, era or demographic will claim at least one should supplant Jordan as No. 1 (the MJ-LeBron debate has been hot for nearly a decade).

Are today’s athletes better, or are we just more aware of them because of technology?

The world’s a smaller place now than in 1999, when the internet was only beginning to take hold. It’s now impossible to ignore the great competitors from other continents.

Tennis has always been an international sport, which makes Williams’ sustained supremacy even more impressive.

She checks the boxes on all of the categories listed above. Her longevity is especially eye-opening, considering how quickly tennis chews up many talented players.

She’s definitely been dominant. Maria Sharapova tried, but Williams hasn’t had what you could truly consider a rival — except for her sister Venus at times.

Chris Evert had Navratilova, Bird had Magic, Ali had several.

Serena Williams’ clearcut superiority over such a long time is in stark contrast to the nearly dead-even battle at the top of the men’s game between Djokovic and Nadal.

Williams has dominated women’s tennis like Jordan did basketball, and Wayne Gretzky did hockey.

But what does the distance between No. 1 and No. 2 during an era mean, if anything?

More sports are taken seriously than 23 years ago.

By the way, that turn-of-the-millennium list included no surfers. Would Kelly Slater make it now? Or, closer to home, how about Carissa Moore?

If we think of world championships as the most objective measuring stick, Moore still has some work ahead to be considered the greatest of all-time in her sport.

Layne Beachley and Stephanie Gilmore are tied for the most with seven each. Moore could win her sixth this season. She doesn’t turn 30 until later this month, so time is on Moore’s side.

What will a list of the greatest athletes of all-time look like 10 years from now? In 20?

Hey, if a horse can make such a list, it’s not hard to imagine an e-sports competitor or two or a pickleball player.

The only thing for sure is that no two people’s rankings will be the same.