I like to think that the FF fans are divided in categories. The most common ones are the ones who can't stand XII and XIII.
I'm a rare species: I like the entire series, and XIII is even a serious contender for my favorite... And the games are fresh in my mind, as I've started a marathon last year and now there is only Tactics, VIII and the first two remaining (and the encounter rate of FFI is pissing me off).
I think my favorite would be FFIX, it's hard to decide. My top 5 is IX, XIII, X, IV and VII (not in this order exactly). My least favorite is FFIII. The game just feels cheap and I swear that the cpu cheats.
I don't have plans to play VIII anytime sooon though... I can't stand the draw system.