I chose Vaan. Primarily because he was the gameplay's main character but also because...

The game's primary plot clearly revolved around Ashe. For me there were points in the game where Vaan (and Penelo) seemingly became irrelevant to the the plot. The game made it clear that they were still very much kids in a story where the conflicts were primarily between adults. However, Vaan does have a story that is told over the course of the game, albeit in a much more subtle fashion.

Given that Vaan becomes the most powerful playable character in the game, his story is effectively one of a boy with tremendous talent who lacked direction and focus until the events of the story gave him a reason to focus those talents toward meaningful goals. It's a story about how sometimes getting out of your "home" or comfort zone is necessary to get out of the rut one is in. In particular it is a message to kids in bad or unfortunate settings or circumstances: it doesn't always have to be this way, you can be better than this...don't just dream big dreams, make them happen. In that sense it is a much more "real" story than the grandiose love stories or the struggles against evil empires that have dominated the series.