It's a debate that has raged on for over a decade now and a whole host of high-profile footballers have provided their personal answer to the question down the years.
To be honest, there's never going to be a universal answer, but it's always interesting to see how other players feel about arguably the two greatest the sport of football has ever seen.
The latest player to provide their take on the never-ending debate is Manchester United starlet, Mason Greenwood.
The 19-year-old is already a first-team regular at the Red Devils and had it not been for injury, he would have represented England at Euro 2020.
Given his age, Greenwood will have spent most of his young life watching Messi and Ronaldo dominate football.
And the young Englishman has sided with Messi in the battle of the legends.
“For me, it’s always got to be Messi,” Greenwood said in an exclusive United fans' Q&A session, per the club's official website. “For me, he’s just on another planet.
“Ronaldo is obviously amazing, a physical, great athletic footballer. But to do what Messi does at his size, what he’s done for Barcelona and winning that trophy for Argentina, he is just on another level.”
Greenwood appears to have been slightly swayed by Messi's magnificent performances during Argentina's successful Copa America campaign this year.
The Barcelona man netted four goals and contributed five assists at the tournament to help his country win silverware for the first time in 28 years.
However, it cannot be forgotten that Ronaldo has thrived just as much on the international stage with Portugal.
The 36-year-old was the catalyst for his nation's triumph at Euro 2016 and he also helped them win the inaugural UEFA Nations League.
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Basically, we're trying to point out that picking a clear winner between Messi and Ronaldo is pretty much impossible.
Fair play to Greenwood for his answer, but we simply cannot separate the superstar pair.