Kaka played alongside Ronaldo for the Selecao and briefly during the latter's stint at AC Milan, which was blighted by injury problems.
Although the 32-year-old has featured in the same Real Madrid side as Cristiano Ronaldo and faced Barcelona's Lionel Messi, he maintains his compatriot could execute a move with greater speed and finesse than any other player.
"For me, the best players are those who are able to think of a play and execute it quickest and in the best way possible, and Ronaldo Nazario has been the best at that," Kaka, who began pre-season preparations with MLS side Orlando City on Friday, told reporters.
"The speed of thought that he had - and the speed he had to carry out his actions - were perfect. It was something amazing.
"I've thought a lot about other players before. I think he was a phenomenon."
Kaka, who claimed the Ballon d'Or in 2007, was part of Brazil's victorious 2002 World Cup campaign alongside Ronaldo.
