The still-active duo, who have 11 Ballon d'Or awards between them, are widely viewed as two of the greatest players the sport has ever seen.
"He (Ronaldo) was the best," Roberto Carlos told Goal, via Alex Kirkland of ESPN. "There will never be another Ronaldo 'Fenomeno.'
"Not Neymar, Cristiano, or Messi. Ronaldo is unique. He's the best. 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."
The two Brazilians, who won the World Cup together in 2002, were also part of Real Madrid's famed "Galactico" era in the early 2000s.
The left-back isn't alone in his appreciation for his compatriot's on-pitch qualities.
Fabio Capello, who coached both players during his time in the Spanish capital, said Ronaldo was the "biggest talent" he ever worked with.
"And at the same time, it was him who caused me most problems in the dressing room," Capello told Sky Sport Italia, as translated by ESPN.
"He held parties, anything. Once, (Ruud) van Nistelrooy said to me, 'Boss, the dressing room smells of alcohol.' Then when Ronaldo left (in January 2007) we started winning. But if you're talking about talent, he was the best, without doubt," Capello continued before stressing the pair have a good relationship.
For his part, Van Nistelrooy denied making such a statement, saying on Twitter that there was always total professionalism within the Madrid dressing room.