It was my favorite saga in my times of extreme vice. I played all of them until XII, then I didn't have time to follow the ones that came out later.
I agree with VII being in first place, it's a fascinating story but the game doesn't give you the full story. In order to get the whole story you need Final Fantasy Before crisis (NS), Final Fantasy Crisis Core (PSP), Final Fantasy VII (PS1) and Final Fantasy Advent Children (Movie).
I would leave the second place to FFXII for having such a large, colorful and good scale map and a relatively interesting story.
FFIX was very entertaining at the time, it could have position 3.
Then there are two others that I don't know what number they correspond to but I liked them a lot, these were Final Fantasy Agito and Final Fantasy Tactics.
Well you may not have noticed but this is only rank 10-6! I've got 5-1 coming too, so I think my general rankings are a bit different to the average which always puts VII as number 1. You raise a good point about VII and the other material but I always dismissed advent children as just fan fiction. I mean, I watched it SO many years ago and I don't recall it being particularly enjoyable.
And... too poor to own a psp back then XD
Honestly you havent missed out much! No 'bad' games, but none "incredible' either