Yuna is in charge. Auron and Tidus don't make the decisions.

Yuna is the focal point of FFX, plot-wise and story-wise. Without her, there is no change, no reason to continue. She is the Executive. She is in charge.

Auron does not make the decisions. While he is the mentor, and knows the most, his actions are mostly about fulfilling his own promise to a friend. He does very little decision making. In fact, he confirms this, and relinquishes any authority that he could have as a decision maker, stating that "as long as the pilfrimage contnues, Yuna may choose to do as she wishes." If that weren't a clear showing of his lack of authority, well, just... *rambles on in random letters and numbers*

Tidus is not the leader either. He is more of the catalyst of the adventure. If you had him alone, nothing would happen. when you add him to Yuna, everything happens. However, as you progress further into the game, he does develop more of the changes to what one could define as being a leader. This is shown at the duel with Seymour at the top of Gagazet. Being a leader means knowing when to get help.

The entire time, he is conspiring with Rikku to find a way to save Yuna, but when Seymour attacks, he instatly knows to send her to get Auron, who knows how to deal with the situation. Also, after Yunalesca, he seems to take on much more of a coordinating role, pulling the rest of the team together to finish something. Yuna has lost most of her authority here, as she is no longer accomplishing what she originally intended to accomplish, but she does mean to finish up the BIG picture of defeating Sin, which she genuinely intended to do, just not in this way.

The concept of balance comes into play here. Maxx Power is right, with the whole consensus in the latter parts of the game, but it would seem to me that Tidus is the one who is, only just BARELY in charge. He also makes the many of the decisions while inside Sin. He has no problem with losing his power. I doubt that he even knows he has it. But for 75% of the game, Yuna is most certainly in charge.