Messi is the reigning FIFA Ballon d'Or champion and many feel he is likely to retain the award for 2011 after helping Barcelona to success in La Liga and the Champions League.
However, Ronaldo was handed Marca's Pichichi award earlier this month for his record 41 goals last season, and Real Madrid team-mate Kaka, the winner of the Ballon d'Or and FIFA World Player of the Year awards in 2007, feels the Portuguese has the edge.
"Messi is extraordinary," Kaka said. "To say who is the best player in the world is relative because a lot depends on what you do for the collective.
"He and Cristiano are the best in the world, but in today's game, in the modern game, Cristiano is the most complete player in the world. Cristiano can score goals with his left, with his right, with his head and from set-pieces. Messi is excellent with all the great things he has, but Cristiano is a more complete player than him."
He added: "Cristiano is the best player that I have played with. I have played against Zidane, but I did not play with him. Ronaldo is the best for his genius and his speed of thought. Ronaldinho is very close to that level."