The sad thing is, Square have said 10,000 times that Vaan is NOT the Main character, and it's annoying and repetitive when people say he is, or even assume it.

Anyway, I would say Ashe is most deserving of the main protagonist title, cause it's basically revolved around her story, and how she must overcome difficulties, also, if you ask me, Ashe went through the most character development. Why? Cause I feel she was close minded to the people of Dalmasca, and didn't realize their struggle, but was only concerned with reclaiming the throne for herself. Later on, she realizes that her friends were a mixture of commoners, and prestigious figures, who risked their lives countless times to protect her and restore her to the head of the throne. She probably sensed the moral of her journey at that point, and it was to serve Dalmasca as a queen, sympathize with the people, and not only focus on her self-needs.