Ronaldo earned a standing ovation from the home supporters after his wondergoal in Madrid's 3-0 quarter-final first-leg win at 10-man Juve.
The five-time Ballon d'Or winner broke the deadlock within three minutes before producing a moment of magic just past the hour-mark when he sent a bicycle kick beyond a helpless Gianluigi Buffon in Turin.
Speaking to the media post-match, Juve head coach Allegri told reporters: "He has been the best forward in the world in recent years. He has a clarity that nobody else has.
"The fans showed class [by applauding]. Football is a show and when there is a moment like this one, it is correct to applaud.
"The best two teams are Real Madrid and Barcelona because one has Ronaldo and the other has [Lionel] Messi, which is a major advantage."
It was a rematch of last season's final, which Madrid won 4-1 in Cardiff, but Juve did not fare much better this time around.
Juve responded well to Ronaldo's early opener, unfortunate not to equalise prior to half-time, but the Italian champions fell apart after the interval.
Ronaldo added a second and Marcelo completed the scoring, while Juve star Paulo Dybala was sent off for a second bookable offence.
It was Juve's first loss in all competitions since November, and a first at home to Madrid in Europe since 1962.
"Up until their second goal, we had defended very well and allowed Real very little," Allegri said. "There was this misunderstanding and we were made to pay. All three goals could’ve been avoided, really, but in these games if you don't have a little bit of luck and the opposition do, it becomes difficult.
"Playing against Real Madrid and thinking of keeping two clean sheets is just about impossible, so you have to try to score goals. The team did well, Dybala had that chance, but Ronaldo scored with a half-chance. He has found the net in his last 10 Champions League games, so it's not a surprise.
"The 3-0 result is disappointing, above all as Sergio Ramos will be suspended for the second leg, so with 2-0 we could've had some hope. We've got the Scudetto and Coppa Italia [final against AC Milan] to play for, so we'll think about that from tomorrow.
"I have nothing to reproach my players, as we were up against the best centre-forward in the world and paid for a few mistakes. We have to lift our heads, think about the Serie A race and then go to Madrid to play our game."