well, spoilers ahead...
FF1 was a weird game. i can't believe i finished it.
FF2 , aka FF4, was pretty good, but not quite up to the standards of...
FF3! aka FF6, was probably the best in terms of its technology those days and standards in storyline and such.
FF5, now, this was interesting too
first time job-class was introduced, i think.
FF7, i DID see my cousin play it, but ultimately never had much chance to play it. i believe it would be very good. i'll have to play.
FF8, although the magic gathering thing was pretty boring, still was fun to cast ultima all the time
FF9 same story with FF7. i don't know much about this game. not even if its for the original PS or PS2.
FF10 and X-2, i don't have a PS2. i played it, and seems interesting.
but overall, i say FF3 was the best.