Messi scored twice in Sunday's final against France which ended 3-3 after extra-time before Argentina triumphed 4-2 on penalties.
The triumph gave Messi the one trophy missing from his illustrious career which has seen him win it all at club level with Barcelona and Paris Saint Germain.
The 35-year-old has often been measured against legends of the past, Pele and compatriot Diego Maradona, as well as Cristiano Ronaldo and his lack of a World Cup trophy always counted against him.
Now that Messi has his hands on football's greatest prize, Guardiola has no doubt who the greatest of all-time is.
"Everyone has their opinion, but nobody can doubt he's there with the greatest of all time," Guardiola said.
"For me, I've said many times: he's the best.
"It's difficult to understand how a player can complete what he's done in the past 50 or 70 years.
"The people who saw Pele or [Alfredo] Di Stefano or Maradona, the people can say 'my favourite', but these opinions are sentimental.
"On the other side, if he wouldn't have won the World Cup, the opinion about what he has done for the whole of football and my opinion of how he is as a player wouldn't change absolutely anything.
"But it's normal that it depends if you win [how] you are evaluated. Of course, for him, it's the final cherries on his incredible career."
