I think the answers to this question are going to vary a lot with age. Younger gamers will probably lean toward the newer, 3D FF's, like FF7. Older gamers like myself may point toward FF2 or FF3 (US).
I personally enjoy FF3 the most. Though much of the game mechanics, style and graphics were setup by FF2, it was FF3 that really came out with the things that we've come to expect from FF games - like in-depth plot and superb character development. I feel like FF3 was the most non-linear of the games. By that, I mean that you had a great deal of freedom as far as where you can go and in what order you want to do things. I felt like the later FF games, especially FF7, were too confined; I felt like my hand was being held throughout the game and that I was being guided in one specific direction.
I enjoyed Tactics a great deal as well. Chrono Trigger too.