Zidane for me, all day long. Iniesta is a brilliant player, but I think he's a notch below Zidane in almost every department. IMO Zidane had better first touch, better technique, better all round skill, I think they'd be pretty even in terms of creativity and reading the game but I'd say Zidane was also a better finisher, especially from long range. Also you have to factor in Zidane's heading ability.
Of course my claims could easily be refuted, it's all subjective and down to one's own personal opinion. There is no doubt that both have really stood out as amazingly talented players.
One other thing I'd like to mention in this debate, is it probably comes down to style, why a lot of people are picking Zidane. Iniesta, though brilliant to watch, seems a lot more clinical, more concise, with every quick footed movement finely measured and engineered.
On the other hand Zidane was so relaxed, so languid, his lazy style of almost caressing the ball with his feet, moving it so fluidly, you'd almost think he had the ball on a string.
He used an almost imperceptible shift of his body weight to let defenders fool themselves. Even when running with the ball, like a gazelle galloping across the open plains, by the time his marker had used his pace to catch up with him, the slightest change of direction would leave them chasing shadows.
Combine this with his amazing ability to pick a perfect pass or strike a thunderous shot, and I think we're talking about a near complete player.
I know it's a cliche, but watching Zidane was like watching poetry in motion.