Well, I'm gonna go out on a limb and say they've all been the "player of the season". Every last one of them has had something good to contribute to this amazing season. Even Hleb. People rag on him, but let's not forget, he was the guy who passed the ball to Henry, who passed to Messi, who chipped Iker for the icing on the cake against Real Madrid in December. You can say we probably already had the 3 points, and that it was really just Henry and Messi, but if Hleb had passed to anyone else, I doubt we would've scored that 2nd goal, and who knows what could've happened afterwards.
So that's my take. But if I had to choose the player of the season, I'd go along with mostly everyone here and say it's either Messi or Iniesta. And notable mentions are Henry and Yaya. They've improved immensely.