Ven, Terra, Aqua. Despite what Nomura says, I think this is the better order. Ven's game is easiest and Aqua's is definitely hardest. After all, she has to fight one of the hardest main game bosses in the whole game in her final battle.
SPOILER!!:
And she has to fight Terra's final boss in two phases as the final, final battle.
But that's my order. Really, TVA is best plot-wise, but VTA may be better for learning the game gradually. Really, though, its your decision.