The Argentine has four goals in his first three games this season, while Ronaldo is yet to score for new side Juventus.
Speaking to 'Sport', he explained: "He (Messi) is the best in history, no doubt.
"Nobody has done what Messi's done. I hope he plays 20 more years.
"Those who love football would love to have him here (Barcelona) for much longer.
"I think when he leaves, nobody will be able to take the No 10 of Leo.
"For me it's not a question over who is more complete, it's a question of taste. I prefer Messi's style."
Ronaldinho, 38, also wishes he had the chance to play with fellow countryman Philippe Coutinho.
"There are a lot of players who I didn't coincide with and I would have liked to play with them.
"Coutinho is one of them. Iniesta and him are great players. They each have their own style, but I think the Coutinho will be one of the big names in world football.
"He already is, but I think that he will be more still."