The Portuguese winger has identified the German champions as the team to avoid after watching their 12-1 aggregate demolition of Sporting Lisbon in the last round.
"Bayern Munich are very strong. I watched them play against my old team, Sporting, and I was very sad because I didn't expect Sporting to lose so heavily," Ronaldo said.
"They have mature players, strong players and quick players. Franck Ribery and Miroslav Klose are very impressive."
However Bayern remain way behind Liverpool, Chelsea, Barcelona and United as favourites to lift the Champions League trophy in Rome in May.
It is Benitez's side who have triggered unrest on the continent, with Johan Cruyff insisting they are the most dangerous opponents, though Barcelona coach Josep Guardiola believes Arsenal are the team best suited to beating the Spanish league leaders.
But it is United who are most fancied to win the competition and Ronaldo
is confident Sir Alex Ferguson's side are well-equipped to become the first team to retain the trophy in the Champions League era.
He said: "We're positive about all the competitions we're involved in and we want to win them all. It would be great to achieve that. Last season we won the league and European Cup and this we have the opportunity to do even better. We'll continue working hard and we believe we can do this."