Carlos played alongside Ronaldo for club and country, winning the World Cup together for Brazil in 2002 and also enjoying success during the 'Galactico' era at Real Madrid.
Ronaldo won a joint-record three World Player of the Year titles during his career, but that mark has since been surpassed by both Messi and the Portuguese Ronaldo, who have won the Ballon d'Or six and five times respectively.
However, Carlos is in no doubt that the Brazilian Ronaldo remains "the best", even ahead of the greatest players in the world today.
"Ronaldo stood out in training. He was the best. There will never be another Ronaldo 'Fenomeno'. Not Neymar, Cristiano or Messi – Ronaldo is unique," Carlos told Goal.
"I think in our generation it was harder to score goals. It was more physical back then and attackers were less protected. But Ronaldo could do everything.
"I knew how unbelievable he was when he arrived but a lot of people at Madrid didn't. He surprised a lot of people. I remember 80,000 people were very happy and pleasantly surprised by him."
Ronaldo scored 104 goals in 177 games for Real Madrid from 2002 to 2007.