I think there's a difference between aesthetics and effectiveness. People tend to hold Zidane in incredibly high regard because of the aesthetic value of his game. He was certainly an effective player as well, but the reason he is thought SO highly of is how graceful he looked and such.
And I think aesthetic value is important when one thinks of who their favorite players are to watch. However, it is not important when talking of effectiveness. If two players do the exact same things, with the exact same results, but one looks great doing it and the other looks awkward, they are actually equally effective players and IMO equally good.
If you judge players based on effectiveness, Zidane does not look nearly as good. He was quite inconsistent, and while he was a good playmaker who could boss a game, he was not actually an exceptionally decisive player if we judge him compared to other legends. A lot of his great moments involve having some amazing first touch or something but not doing anything with it after that. This shows skill, but is less of an indicator of effectiveness (although an amazing first touch can allow you to keep possession when a lesser player would not have, so it can be effective, even if it's not really decisive).
To some degree, I think Iniesta gets too much credit based on aesthetics too. He looks like he is gliding across the pitch, and people like to watch that, but looking like you're gliding doesn't make you a better, more effective player. Similarly, a lot of his great moments involve keeping possession in tight spots, but then playing a simple non-dangerous pass afterwards. This is effective in the sense that it allows his team to keep possession and may create some extra space, but it typically is not a decisive moment that leads to any sort of goalscoring chance. Contrast this with Messi's dribbling, which also typically brings defenders towards him, but which very frequently ends in some kind of direct, decisive pass. Similar to Zidane as well, Iniesta is not entirely consistent.
So I think Iniesta and Zidane are both a tad overrated. However, I think Iniesta is the better player. While Iniesta is not insanely consistent, there is something to be said for the fact that, at the very least, he helps Barcelona boss the midfield EVERY single match. He may not produce magic every match, but he is always an important part in Barca having the majority of possession and setting up shop in the opponent's third. Zidane did not produce magic every match, but also was not bossing the midfield every match either. Furthermore, Iniesta is a better dribbler than Zidane (not that Zidane wasn't a great dribbler), so he can be more decisive.
I think Xavi is better than both of them. I think it's odd that Iniesta gets more plaudits than Xavi. Xavi's game is less magical looking, but is more effective. He absolutely controls the midfield of EVERY match he plays in a way that neither Iniesta nor Zidane could dream of. No one has ever averaged as many passes a match as Xavi. One can say it's because of the Barca system, but Xavi IS the Barca system. Furthermore, I think he's a more decisive player than either one if you look at his exceptional ability to give a through ball. He does it less now, since Messi has become a fantastic passer and the team has decided it's better to have him try a decisive pass further up the pitch when players are drawn to him and others have lots of space. However, if you look back to 2008-2010, Xavi was making a laughably large number of ridiculously good through balls.