Messi has established himself as one of the all-time greats in football – becoming Barca's leading scorer in their history and claiming a record six Ballons d'Or.
But Barca great Iniesta – now playing for Japanese side Vissel Kobe – ranks former teammate Messi as the number one.
"He makes the difference in every way," Spanish veteran Iniesta, 36, told Ole.
"He is as capable of scoring a hat-trick as he is providing a hat-trick of assists, if it looks good watching from the stands, it looks even better when you're close up.
"For me, he is number one because of that, he can make the difference at any moment, although he can't be the whole team."
Since emerging at Barca in 2004, Messi has helped deliver four Champions League titles, to go with 10 LaLiga trophies and other silverware.
Iniesta said Barca should have been able to celebrate more Champions League success with Messi at the forefront.
"With the team at Barcelona and with Leo there, Barcelona surely should have been able to win some more Champions Leagues," Iniesta added.
"Soccer is like this though; you play tough opponents who also have talent.
"It's important that when you do win things, you savor them."