Well, the answer is FFXII. It's a little more complicated than that.
I mean I want to say FFXIII is the last one for me and shrug. It's not the best FF, but it's not a bad game and there's a lot of things this FF-team did right in this generation that's actually pretty astounding considering this is their first game on the PS3 and 360.
But I'm going with XII because FFXIII only showed how ahead of its time FFXII was.
I also want to say FFX, because I did enjoy that like I liked VII-IX, and XII didn't have that Final Fantasy feel. So that was the last game in the series to have that FF vibe to it AND be good.
But I'm still going to give it to XII. Like I said, in 2010, nearly 4 years later, I have to say DAMN that game was waaay ahead of its time. With a slight upscaling this would look and just be better than most ps3/360 games made today, while keeping that same feel. I mean XII was basically a bold answer from Japan to KoTOR, and that game's progeny like Mass Effect are what the kids playing RPG's today worship. I mean it had seamless battles (XIII didn't), an actual world with tons of places to explore from nearly the beginning of the game, you can customize the battles deeper than Paradigms, and it looked good enough to pass off as (or better than) an early PS3/360 game.
All I'm trying to say is they should've split the XII team into two like they did after FFVI and have them both work on "next-gen" Final Fantasies, because that was clearly the team to go with.
Hopefully with Matsuno, Sakimoto, Minagawa and Yoshida all working on Tactics Ogre they may actually come together and do a PS3 game. Seriously, after thinking about all this, that's one of the best teams in gaming they should be seriously thinking about a game right now.