Wow, this is a toughie...if I'm being subjective, I'd probably say the best is X for being the most emotional game I have ever played as well as for ushering the series in to a new generation in such a revolutionary and successful way, but honestly I found the game to be somewhat lacking in a couple practical aspects. Certain parts of the game that could have been done better or done away with made it feel like a chore to slog through in order to get more of the story (e.g. the half-billion Seymour fights).
If I'm being objective, which I was in my actual voting on the poll, so I'd have to say XII is the best FF game. The battle system is a fantastic blend of other systems in one, the new license and gambit systems are a delight to use (once you get used to them), the story runs fairly deep, dialogue is simply fantastic in an archaic sort of way, the characters all have a LOT of character (with a few minor exceptions), the graphics are about the best on the platform, and the music is a nice blend of old and new as well. Overall, it seems like XII just tries to take all the best elements of other games, and the result is an extremely solid-feeling game and universe. I think SE knew with XII that they were releasing it to the competition of not just other games, but an entire new platform (as the PS3 was to release less than a year later and it had already been shown around plenty of times) and so they really put their best foot forward.
If I'm being objective, which I was in my actual voting on the poll, so I'd have to say XII is the best FF game. The battle system is a fantastic blend of other systems in one, the new license and gambit systems are a delight to use (once you get used to them), the story runs fairly deep, dialogue is simply fantastic in an archaic sort of way, the characters all have a LOT of character (with a few minor exceptions), the graphics are about the best on the platform, and the music is a nice blend of old and new as well. Overall, it seems like XII just tries to take all the best elements of other games, and the result is an extremely solid-feeling game and universe. I think SE knew with XII that they were releasing it to the competition of not just other games, but an entire new platform (as the PS3 was to release less than a year later and it had already been shown around plenty of times) and so they really put their best foot forward.