Ronaldo won numerous individual awards during his career, including the Ballon d'Or in 1997 and 2002, while also winning two World Cups with the Selecao.
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"The best player I have ever played with? That's Ronaldo, il Fenomeno," the AC Milan star told Tiki Taka.
"The other Ronaldo - Cristiano - probably makes my top five, but I have seen il Fenomeno do things that nobody else has ever done."
Kaka then went on to discuss the situation of Mario Balotelli at AC Milan in the wake of recent criticism aimed at the striker.
"Mario is a great guy, but he has to understand that he has certain responsibilities. That's the case with Milan and with Italy's national side.
"He is an example for a lot of people and he has to understand his status as a role model and learn to deal with it."
Kaka, 31, has netted once in five Serie A appearances for Milan since his return to the club from Madrid over the summer.