Hmm... I'm going to order the games in the series from favorite to least just for the hell of it...
1. VII (The greatest, deepest storyline of any FF, in my opinion. Not to mention the soundtrack is embedded into my brain, along with most of the dialogue.)
2. VI (Though Kefka is vastly superior to Sephiroth, and I love the characters more than VII's.)
3. IX (Played since I was ten. So much nostalgia, and great characters.)
4. V (If only for the job system. I want another FF game like this.)
5. X (Annoying lead hero and annoying lead villain, but everything else was great. And I love Blitzball.)
6. IV (Oh, Kain. How easy you are to mind control.)
7. XII (Vann and Penelo have absolutely no use to the story and Vayne is pretty generic, and though the battle system wasn't really my thing, I can't deny this is a great game.)
8. II (The first with a compelling story & characters)
9. I (The game that started it all)
10. VIII (I think if I played through again today I would enjoy it a lot more. I was spoiled with VII and IX)
11. III (I could only ever play the DS version of this. Not a bad game, but I can't rate it fairly.)
12. XI (God's bane to a world of sin)