The 33-year-old has hit 21 goals in his last 11 Real Madrid appearances and he also struck a late brace to help Portugal come from behind to beat Egypt last week.
Ronaldo beat Buffon twice in last season's Champions League final in Cardiff and the Madrid star will face the veteran goalkeeper again in the first leg of the quarter-final on April 3.
"Over time he has learned how to improve himself as an athlete," Buffon said.
"As a player, he spends less energy with respect to what he did previously.
"I admire him a lot because he is a pragmatic person, lucid in everything he does and what he says.
"I think that in recent years he has shown that he is very intelligent, because he has changed his way of playing.
"He is also lethal, everything he does is at 100 percent and in front of the goal he is an assassin."
Alcune volte abbiamo vinto noi, altre volte abbiamo perso. La verità è che quando si perde con determinate squadre, composte da campioni simili, non hai i rimpianti e le frustrazioni che, di solito, accompagnano le sconfitte. Spero valga anche per loro!Vincano i più determinati! pic.twitter.com/lzhvQWTCO0— Gianluigi Buffon (@gianluigibuffon) March 16, 2018
Buffon, who was speaking in a Facebook Live question and answer session, was asked about former team-mates Filippo Inzaghi, David Trezeguet and Alessandro Del Piero.
"They are three icons of world football," Buffon said. "Inzaghi and Trezeguet, unlike Del Piero, were real penalty area assassins.
"Ale [Del Piero] had some extra quality, he needed space to demonstrate the technique he had, and he could enchant big crowds. Even just training with them allowed me to know what champions they were.
"I really struggle with these things, I've played with many champions and I don't want to forget anyone.
"I can mention players I haven't played with - as a trequartista [advanced playmaker] [Zinedine] Zidane was the absolute best. I never played with him, I came to Juve and he went away."