As weak as a main character as he was, I'd have to say Vaan. My criteria is usually just as simple as the permanent character who you control first/most (not including prologues), or who is the default overworld map character.

I think Ashe works better as a main character, but I still think its Vaan. Vaan was almost like an afterthought, but I still think he had enough invested in the story to be the main character.

Basch was originally intended to be the main character, and I thought he would've worked better. But somebody at SE thought they needed to have a main character more towards the age of their main demographic.