If you are talking about who is the most significant character in the story, then its certainly NOT Yuna. Without Tidus, and more particularly Auron, Yuna would either have repeated the usual summoners' quest of defeating Sin with the Final Aeon or she would have given up.

As the plan to kill Yunalesca was Auron's and the motivation to change things was Tidus', either could be the main character. Basically though, I agree as you actually play Tidus, its him.