It is a simple question, but one that provokes much thought, endless debate and loads of arguments, which often times even gets angry.
And it is a question that can be asked in any sport.
Who is the greatest?
When The New Paper posed that question to Lleyton Hewitt, the Australian tennis star and two-time Grand Slam winner (Wimbledon in 2001 and the US Open in 2002) seemed uncomfortable picking a name.
Perhaps he was thinking of his countryman, Rod Laver, who ruled in the 1960s and still is the only man to win two calendar Grand Slams.
Maybe he couldn't separate his fellow Aussie from the likes of Pete Sampras, Bjorn Borg and John McEnroe.
Or those who ruled the game in this millennium.
Put Hewitt on the spot, though, and he will stick his neck out.
"I can't rate your Lavers, your (Ken) Rosewalls but, right now, it's Roger Federer," the 34-year-old said, when he sat down with TNP for a chat at the Australian High Commissioner's residence yesterday.
"People will ask how he can be the greatest when he has a losing head-to-head record with another guy who's playing in his generation (referring to the Swiss player's rivalry with Rafael Nadal).
HARD TO COMPARE
"But it's a tough one to weigh up, and he (Federer) has still won the most Grand Slams.
"If Rafa ever passes him, then he could go down as the 'Greatest'. But right now, I'd say Federer."
Hewitt is in town for the Singapore launch of Swisse, Australia's top multivitamin brand which he endorses.
He is one of the faces of Swisse, along with Aussie cricket legend Ricky Ponting and the Wallabies, who all use the brand's range of vitamins and supplements.
While he feels 33-year-old Federer pips 28-year-old Nadal in the best of all-time stakes, Hewitt sprang a surprise when he revealed who he thought would rule tennis in the immediate future.
The Aussie singled out a potential rivalry between two 27-year-olds - world No. 1 Novak Djokovic and No. 4 Andy Murray.
With Federer ageing and Nadal increasingly bogged down by injuries, Hewitt said: "I guess the next three years (will feature a Djokovic-Murray battle).
"But Roger's still got a shot, especially on grass at Wimbledon.
"And you can't write Rafa off at all and, on clay, it's very hard seeing anyone beating him over five sets."
Much has been made of the dominance of the "Big Four" in men's tennis over the last eight years.
Hewitt gushed about Nadal being "nearly unbeatable" on clay. He praised Murray and Djokovic for their athletic ability, having "amazing" on-court movement, and Federer being "such a natural all-court player".
But he found it tough to describe the era as the best in the men's game.
"It (the current era) is certainly up there," said Hewitt. "It's very hard to compare some of the greatest eras to each other.
"When I grew up, there was (Ivan) Lendl, (Boris) Becker, McEnroe, (Mats) Wilander, (Pat) Cash, (Stefan) Edberg, (Jimmy) Connors... there were a lot of great players then as well.
"Then (in the 1990s) you had Sampras, Agassi, (Jim) Courier, (Michael) Chang.
"Obviously these current guys, the 'Big Four', have been pretty amazing in terms of how dominant they've been.
"Obviously for me to play against these guys and to know first-hand how good they are, it's easy for me to say 'them'.
"But growing up, I idolised that other group and dreamt of one day being out there and competing like they did.
"So I don't know. It's hard to say."