The order I will probably play is in Terra, Aqua then Ven. I want to finish is off strong cause I think Ven is the most important out of all of them..and I don't think Aqua will be that important :X. However, I hope Aqua turns out to be a important character.
My thoughts exactly, Ventus is still in the Sora/Roxas category thus making him likely the most important. Best for last and all that.