It's a question sports fans have often asked themselves, and after analysing 600 of the biggest sportsmen and women in the world, ESPN believes it has an answer.
In order to produce its World Fame 100 list, the media giant used three metrics to determine popularity:
Once all of these factors were taken into account, ESPN was able to whittle the 600 strong list down to a top 100.
Scroll down to see the 18 most famous athletes in the world, ranked in ascending order:
The UFC fighter is renowned for his work in mixed martial arts, but the Irishman only fought once last year — and it was in a boxing ring rather than a UFC octagon. McGregor's loss to Floyd Mayweather made him $100 million richer, and his alleged attack on a UFC team bus last month made him even more infamous.
Chinese swimmer Yang is a gold medal magnet and his performances in 2017 continued to attract titles. His highest social media profile has 32 million followers and he was the only athlete to grace the 2017 Forbes China Celebrity List.
The American golfer is "the real deal," according to ESPN. Last year was a standout period for Spieth as he won The Open, a victory that ensured he had won three of golf's four majors at 23 years old.
The US Masters continues to elude McIlroy's grasp as it is the only golf major he has not won yet. The Northern Irish athlete has made $35 million in endorsements so far and turns 30 next year.
The retired boxer was the most searched for athlete on ESPN's list. He last fought in August 2017 and defeated UFC fighter Conor McGregor in the 10th round of their boxing rules contest, a fight ESPN called a "farce."
Tennis star Djokovic may have fallen from the summit of his sport, but he retains a good social media following, is a UNICEF goodwill ambassador, and has a foundation that supports childhood education projects.
The American tennis player took a hiatus from sport to focus on her pregnancy last year. But despite being away from the game, she remained the most visible women's player in tennis. She graced many magazine covers, wrote an article on the gender pay gap for Fortune, and married Reddit co-founder Alexis Ohanian.
Indian cricket captain Virat Kohli was named in Time's 100 most influential people in the world with good reason — he is the 2017 ICC World Cricketer of the Year and is married to Bollywood star Anushka Sharma.
The American golfer may be 48 in June, but that has not deterred his success, or commercial viability. He won the Mexico Championship in March and has made $50 million in endorsements.
According to ESPN, Golden State Warriors point guard Steph Curry is "the most popular player in the NBA." His biggest social media channel has a following of 19.4 million and he found even bigger fame when he said he didn't want to go to the White House to celebrate last season's championship.
President Donald Trump later rescinded the invitation.
Spanish tennis player Rafa Nadal is enjoying a career renaissance. He finished 2017 as the ATP world ranked number one, and continues to compete with arch rival Roger Federer.
The basketball player has had a busy year. Kevin Durant swapped Oklahoma City Thunder for Golden State Warriors and founded The Durant Company, which invests in tech startups.
The American golfer's DUI arrest "might have been his lowest point" and his back injury even jeopardised his playing career, but Woods is "fit… healthy, smiling, and thankful for an opportunity to extend his career," according to ESPN.
The 20-time Grand Slam tennis champion has undergone a "career reboot," according to ESPN. After a six month layoff from injury, Federer won two majors and finished 2017 with a world number two ranking. He has also given considerable time to his Africa foundation.
Brazil forward Neymar swapped FC Barcelona for Paris Saint-Germain in a $263 million deal last summer and will be a key player for his country in the upcoming FIFA World Cup tournament in Russia.
The FC Barcelona forward has a social media channel followed by 88.1 million fans on his biggest platform — but that's over 30 million less than Ronaldo. Messi's name, though, is more widely searched on the likes of Google.
Cleveland Cavaliers small forward James is the NBA's best player, according to ESPN, but "it's his social commentary that has set him apart" in this year's list. This is because he called President Trump "a bum" in a tweet that was retweeted 650,000 times.
Real Madrid and Portugal national team striker Ronaldo is the most famous athlete in the world, according to ESPN. He is the Champions League all-time leading goalscorer (120), has equaled Lionel Messi's record of five Ballon d'Or trophies, and his biggest social media platform has 121.7 million followers.