IMO, the GAMEPLAY was at its prime in X-2. It was the best gameplay feeling and acting game (international really took the cake).
XII had the best world design and probably most intricate story around it. It was really the most "interesting" type of RPG out there. A lot of great concepts went into it, but the game had a few faults, however, I rate it as the second best FF I've played.
X, however, is the all-around best in the series for me. I thought it did the best job of everything, and it had a great depth to it and all that stuff. People say it had no role elements and stuff of that sort, but if they played it a certain way, they'd see it was probably the most versatile game of them all. You can't really tell how great of a game it was until you've sent hours and hours on it.
VII was overrated in my many ways. I'd certainly put it up there in the great 5, but never over it.
I didn't like 9's design, so I won't comment much on it. It didn't appeal to me. So, I probably never enjoyed it as some others might have. I thought it was pretty bad. 8 was one of those I disliked as well.
All in all, I thought the PS2 ones were the best examples for all three. SE really perfected that system. Bummer they blotched on PS3 though.
FFX-2 = best gameplay around. FFX = best story and overall game. FFXII = personal favorite and probably best setting and stuff. VII = pretty good, but a little overrated in aspects. 8 stunk, and 9 was offsetting to me, but I thought it was rather blend/ average/ or boring. XIII was awful. Had high potential, but totally bombed since no one bothered to organize it or plan it properly.