Sports fans could argue forever on this topic, but we've met one man we believe is a serious contender for the title.
Phil 'The Power' Taylor, the 50-year-old from Stoke, is the 15-time world darts champion, who has dominated his sport for 20 years.
Taylor was at the launch of the latest darts computer game when we asked him about being the world's greatest.
Inevitably he was modest, but did reveal the qualities it takes to reach the top.
"It's hard work, dedication and lot of commitment and sacrifice," he said.
"You don't get to where I am by going out and enjoying yourself - unfortunately. You've got to live a little bit like a monk meaning you've got to go to bed early. You've got to do your practise and basically do everything right."
Taylor says he plans to retire at 55, by which time he will hope to have won 20 world titles.
Perhaps then he can stop living like a monk and relax after a career in which he has been relentless in his pursuit of excellence.
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