The Brazil international was widely regarded as the world's best player during that time, but over the last decade, Lionel Messi and Cristiano Ronaldo have set new standards in terms of goalscoring, team success and individual accolades.
The Juventus striker and Argentine talisman have each won five of the last ten Ballon d'Or awards and remain the top two footballers in Europe, but Deco still believes that Ronaldinho was in a class of his own.
"From the outset, it is clear that Leo and Cristiano are the greatest," the Portuguese star told AS.
"Especially because no one thought they could take so many years to this level, but the one that impressed me most despite not being so decisive was Ronnie.
"For me, he will always be the best."
Ronaldinho hung up his boots at the start of the year, bringing the curtain down on a glittering 20-year career, three years after his last professional game for Fluminense in the Brazilian league.
Deco went on to admit that his best years were spent playing with the former Barcelona talisman, as he added: "We met in Barcelona and immediately connected with him, the moments I lived with him are difficult to compare to anything. It was magical."
The one-time Chelsea star also retired after spending his last few years with Fluminense in 2013 at the age of 35, calling time on his career after a bad run of injuries.