I can't say which is "THE BEST", but I enjoyed FFX a lot out of the ones I played. :3 I remember playing one of the early NES ones as a child, but I don't remember liking it well enough to become a fanatic like I did with zelda.
FF12 was....ehhhh. I've played it twice. D: And both times I ended up thinking...well, it was alright, but hurrrrgh. I think it would have been a lot better if it was about Balthier and Fran, or Basch, like it was originally intended to be. I always found it laughable how Vaan was just sort of...there, and even when asked, he had no reason as to why he was just magically there. He was like the FF Aladdin. D: Street-ratting around with some opium-induced old pimp man. (You can't say he's not..he was always surrounded by sluts.)
FF9 was good too <3 Though I didn't actually play it, but I watched my fiance play it :3 And while I never finished it, I did enjoy the Tactics ones.
My favorite gameplay was actually in FFX-2, though I was not fond of the charlie's angels themed storyline at all. D:<