Of the FF games that I own or have played extensively I would rank them like so:
1. VI - The second FF game I played, and I can't count the number of times I beat it. The characters are all really well developed, the story is phenominal and gets you into playing the game alot, and the music is amazing for a cartridge based game. There is not one thing I didnt like about this game at all.
2. IV - The first FF game I ever played. I rented this game way back after it came out not knowing what an RPG even was. I fell in love with the storyline of Cecil and Rosa etc. and ended up buying the game soon after. Played countless hours of this over and over.
3. VII - Again a GREAT storyline and great characters. I tend to get emotionally attached to some characters in these games and when Aeris died unexpectedly I nearly cried...heh.
4. X - I, unlike most, loved the characters and story especially that this game told. The extra stuff before the end add alot of extra playtime as well.
5. IX - Awesome game with again great characters and story...I guess I should just say i get really into storylines and these FF games seem to rope me in better than any other series for some reason. The old-school factor is awesome here.
6. V - Didnt play this a whole lot since I only found it a couple years ago with emulation and all. Definitely has a cool system with jobs and all...which is the best part of Tactics sans the storyline which was phenominal too. I cant add tactics to this list but it's definitely a great game.