I would DEFINITELY have to say Final Fantasy VII I don't know if you've played it, but it's a classic and alot of people say its the best game ever made for the Playstation 1 (yeah its an old game)
I like FFIV, FFVI, FFIX, and FFXII. The rest were overrated garbage. I know! How could I say that about your precious VII!?! Easily. It was boring, drawn out, and way too overrated. None of the characters were interesting, and the story was convoluted. Not to mention, Advent Children was only interesting for the fight scenes.
Just bought Final Fantasy for the NES, must say it was quite the buy. Since I have too short an attention span to finish any Final Fantasy, I'd pick 12 as the favourite based on how far I've gotten. FFIII for DS is an entertaining one... P.S. favourite FF song- Terra's Theme from IV