Messi, 35, has long been considered among the best of all time, but many felt he needed a World Cup to cement his status alongside the likes of Pele and Diego Maradona.
Iniesta, who spent over a decade playing alongside Messi at Barcelona, said the forward's standing in the game will remain the subject of debate despite Argentina's thrilling win against France on Sunday, with Messi scoring two goals in the 3-3 draw and another in the penalty shootout.
"For me, Messi is the best with or without [winning] a World Cup," Iniesta told ESPN at the launch of the new Capitten football boots, the brand owned by the Vissel Kobe midfielder.
"I think the fact he has won a World Cup, more than what other people [think], is a huge source of happiness for himself. Not just for him, but for Argentina as a country. They are always in the running and the fact they've won it in the way they did makes it totally deserved.
"I am sure anyone who doesn't view Messi as the best will find an excuse to keep seeing it that way, regardless of whether he won the World Cup or not."
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Messi opened the scoring against France in Qatar but then watched on as two Kylian Mbappe goals took the final into extra time.
The Argentina captain thought he had won it with a second goal in extra time, only for Paris Saint-Germain teammate Mbappe to complete his hat trick to take the game to penalties, where Argentina prevailed.
"I think it was the best ever final, in terms of goals, emotions, everything that happened in the game, the twists," Iniesta added.
"It was clearly an amazing game for the fans and was worthy of a World Cup final."
Argentina's triumph contrasted with a round of 16 exit for Spain, who last won the tournament in 2010 thanks to Iniesta's winning goal against the Netherlands.
Spain's defeat to Morocco has reopened the debate on the country's style of play, with many critical of their possession-based strategy as they were eliminated before the quarterfinals for the third World Cup in a row.
"I think the model has to stay the same," Iniesta said. "At the end of the day, a model of play is good if you're able to give 100%. The game against Morocco was not lost because of the model or not, but because other things were lacking to win the game.
"I think the idea has to remain. The only thing is every game is different and you have to try and find solutions on a game-to-game basis. From there, the debate will always exist because there are so many small details that can change games.
"Maybe if [Pablo] Sarabia scores unsteady if hitting the post in the last minute, this isn't a debate. We were all disappointed to go out but these experiences serve to learn and to improve."