Messi won a record sixth Ballon d'Or earlier this month, taking him ahead of long-time rival Cristiano Ronaldo.
While the Juventus star has shown signs of decline over the past year or so at 34, Messi – two years his junior – continues to sparkle in LaLiga with Barca.
Despite missing a chunk of the early-season action through injury, Messi already leads the scoring charts in Spain's top flight with 12 goals from just nine starts – Karim Benzema has 11 from 14.
The Argentina star's form has been particularly impressive over the past six weeks, netting 13 times in 10 games for club and country, most recently hitting a hat-trick in Barca's 5-2 triumph over Real Mallorca.
But Ronaldinho, who spent the best part of four years as Messi's team-mate, is uneasy about suggestions the forward is the greatest to ever play the sport.
Speaking at a charity event in Mexico, Ronaldinho said: "I'm glad for Messi because he's a friend, aside from the fact he's been a leader of Barcelona.
"I don't like comparisons because it's hard to identify who is the best in history. There's Diego Maradona, Pele, Ronaldo.
"I can't say that Messi is the best ever, but he is the best in his time."