For me it's FF9, since I've never truly completed 8 (the game froze on a cutscene late in the third disc), I've only watched my cousin play FF7 (though I did watch a fair amount of it, probably 80-90%), and I'm in the midst of finishing FF6, I think I'm pretty close to finishing it (which is pretty good thus far).
FF10 was good, I loved the storyline, but the characters weren't as great. The combat system was nicely done, and everything was decent, but the characters and plotline of FF9 edged out 10. Square fleshed out each character, but I just didn't feel a connection with some of them.
FF6 is entertaining, but I don't like the storyline much compared to the other FF's. Kefka's awesome, though. A villain that's finally villainous.
FF7, well I don't remember 7 well so I can't really say much about it, especially when I didn't play it.
FF8, some characters I loved, others I loathed. Same goes for the story, except there were great moments in the game that edged out the bad for me.
FF10-2 had a potentially good story that could have been a great addition to the FF10 world, but I don't think they played it well, and the whole Charlie's Angels feel...
FF9>FF8>FF10=FF6>FF10-2 for me.