"He's at the top," Giggs said when asked where he ranks Ronaldo in the list of players he has featured with. "Because of his performances and his goalscoring record.
"And he can get better and better. He's only young but he's playing really well for us.
"His goalscoring form is brilliant for us. It's unbelievable for a midfielder to score that amount of goals. He just contributes to the team whether scoring or making goals."
Giggs is stunned his team-mate has already become what he believes is the best player in the world and is ready to pass on all his advice to bring on the 22-year-old.
"I think he's willing to learn off other players and the coaches and management," he added. "The improvement he's had since he came from Sporting has been so quick. I don't think anyone thought he would be, in my view, the best player in the world at the moment.
"Players usually peak in their late 20s so it's encouraging he's playing so well.
"Your all round game can improve, no matter how good you are or how old you are so I think he can improve even more."