The former Barcelona and Real Madrid striker believes the Blaugrana's current South American superstar is "a little bit better" than his Portuguese rival.
The now-retired striker is a big fan of the Portugal international, who has scored a staggering 196 goals in 193 appearances for Real Madrid since arriving at the Santiago Bernabeu from Manchester United in 2009 for a world record 94 million euro transfer fee.
However, Lionel Messi has won the Ballon d'Or at the end of every year his great rival has been in Spain and former Blancos forward Ronaldo can understand why.
"I would say Messi [is the world's best player] because he is a little bit better than Cristiano," the 36-year-old is quoted as saying by Tuttosport.
"But I would also add that Cristiano has had unbelievably bad luck. He would have been the best player in the world in many other eras, but he has coincided with another of the game's greats."
Ronaldo, who played for both Barca and Madrid during his illustrious playing career, quit the game in 2011.