The 34-year-old believes the Barcelona star is the finest player to have played the game and will continue to be so over the next five or six years
The Croatia international was speaking in an interview about who he thought deserved to win the Ballon d'Or and responded by heaping praise on the Argentine.
"Lionel Messi, without doubt. It cannot be anyone else," he told Fifa.
"He has been by far and away the best footballer in the world. He deserves it and I am 100 per cent behind him. He is the best player of all time and if he stays fit he will continue to be so for the next five or six years.
"The things he does right now are out of this world. Of course, he does not play by himself, but he is simply incredible.
"He always finds a way and is always a yard quicker than everyone else. Admittedly, he plays in a fantastic team, but he always makes the difference. If you have a player like that on your side you do not need to fear anyone."
Simunic was also happy to run through who he would select for his Fifa Fifpro World XI.
"I would have Gianluigi Buffon in goal; Jordi Alba, Carlos Puyol, David Luiz and Darjio Srna at the back; Andres Iniesta, Xavi, Messi and Cristiano Ronaldo in midfield and [Radamel] Falcao and Edin Dzeko up front," he revealed.
The award ceremony for both prizes will take place on January 6, 2013.