Eh, may as well post my favorites.
Final Fantasy XII - Incidentally, I initially hated this game because I found it too hard. Ragequit the first time after getting my ass kicked by Garuda. Came back a few months later and played it again. Found an entirely new appreciation for it.
Final Fantasy XIII - Yeah, yeah, linearity blah blah blah don't give no fucks about that.. Looooved the battle system. Even when I was getting my ass kicked, I never stopped enjoying it. Also, the soundtrack. A glorious big budget Hamauzu soundtrack. *drools* The game itself is gorgeous, and I loved all of the character interactions as well. Sazh and Vanille had the best, by far.
Final Fantasy VI - Soundtrack + story + loads to do.
As for least favorites, I don't really have any. Seems like a cop-out, but it's true. I've played and loved every game in the main series, barring the MMOs, as I haven't the time or patience to play MMOs. :V