Played all except XI and XIV. Never saw a point in paying a monthly fee for a game.
First one I ever played was VIII. LOVED IT!
I don't think I can pick one though.
I think all of them are great in their own ways.
In terms of best story, I'd say FFX.
Battle system I'd say FFXIII.
But I think story is far more important, So I guess I gotta vote for FFX!
Edit: I've also been playing IX. The story is A LOT better than I remember. The character development is really great, particularly with Vivi, Steiner, and Garnet/Dagger/Sarah.