Former France midfielder Zidane is widely regarded as one of the best players to have ever played for Los Blancos during his five-year stint at the club, which was brought to an end when he retired in 2006.
However, having seen Ronaldo develop since his 2009 departure from United to the Spanish capital, Ferguson has insisted that the Portuguese attacker is better than most others who have pulled on the white shirt.
"Florentino Perez had all the Galacticos at Real Madrid when he was president first time around," Ferguson told reporters. "He had Figo, Zidane, Roberto Carlos, Raul in that spell. But I don't think any of those players were as good as Ronaldo.
"Zidane was a fantastic player, Figo was a fantastic player. But not as good as Ronaldo. So I think Mourinho's team are still living up to the reputation that Real Madrid can produce the top players.
"Ronaldo has beaten all the records there. He's beaten Di Stefano, Puskas, Hugo Sanchez. So obviously he is indelibly printed in their history now."
Ronaldo will come up against his former employers United when they travel to the Bernabeu for a Champions League tie on Wednesday night.