Neymar became the most valuable player on the planet in completing a world-record €222 million transfer to Parc des Princes last week, after months of speculation over his future.
The forward also cements his reputation as one of the game's most talented prospects, with many expecting him to challenge Cristiano Ronaldo and Messi in the Ballon d'Or race in the years to come.
For Willian, however, the new PSG star still has work to do if he wishes to succeed his former team-mate.
"Messi, Neymar or Ronaldo? The three of them are exceptional, they have a lot of quality and they have their differences. I like all three, but I think Messi will still be the best of the world a few more times," the winger explained to Goal.
"I know Cristiano Ronaldo is close to winning another one Ballon d'Or, but I really like Messi's style. I really like Neymar too, but I really like Messi."
Neymar's decision to turn his back on Camp Nou has not been met with universal acclaim.
Some Barcelona fans have accused him of being a "mercenary", while other commentators fear the world-record fee could further distort the transfer market with outrageous prices for players.
But Willian believes his Brazil team-mate has made the right decision, and that it could not have been an easy move.
"I think every player thinks in a different way. I think it was not easy for him to make that decision to leave Barcelona and go to PSG, but I hope he can do what he was already doing in Barcelona and playing with the same joy in PSG," he said.
"I hope he can be happy in PSG and play in the same way he played in Barcelona. If he's going to be the best in the world or not, it depends on him winning titles in PSG and reach the Champions League.
"If he gets that, he will be closer to the title of the best in the world."