I think it's easier if i give each game i've played a score out of 10
FF1 7/10 (dated by todays standards, but i will give it credit for how mindblowing it was at the time)
FF2 7/10 (very under-rated and a good black sheep, sure the leveling-up may take a while to get used to but once you do it's really fun, the villain is also the best in the series in my eyes)
FF3 4/10 (admittedly i've only played the DS remake, but i really wasn't too impressed by it)
FF4 9.5/10 (probably my favourite game in the series, i just love the characters and story plus in introduced the RTB system)
FF5 8/10 (so addicting to play o.e , i love the job system in it)
FF6 9/10 (a very good game with a great plot, characters and villain, what more needs to be said)
FF7 7.5/10 (over-rated or not this is still a brilliant game, the 3D blew my mind at the time and i loved some of the characters, wasn't a big fan of cloud though
)
FF8 7.5/10 (if 7's graphics blew my mind you can imagine my suprise when i played 8, admittadly i hated this game in the beginning, but after making some progress i really started to enjoy it)
FF9 9/10 (absolutely love this game, loved the art-style, story, music and characters, 4 may be my favourite but this is a close second)
FF10 6.5/10 (not the best but certainly not the worst, i liked the main cast, but the combat wasn't the best IMO)
FF13 5/10 (don't get me wrong i don't think the game is bad, but its just lacking that special spark.... it's a good game, just not a good FF game)