For me if for the sequel or series, I would vote Fabula Nova Crystallis.

But if only one allowed. I vote for Final Fantasy XIII.
Despite it's difference from the past Final Fantasy, like gameplay for some people, and story for other people.
But Final Fantasy XIII will always be my favourite Final Fantasy.
I have Final Fantasy VII Advent Childern for years. My mother said I have it when I'm on elementary school, but I don't even know it was one of the Final Fantasy franchise (is this the right word?) until I watched some Final Fantasy XIII trailer which was 2-3 years ago and join Eyes on Final Fantasy about a year ago.

Well if I count it right, now is 2014, so I don't know Final Fantasy VII Advent Childern was one of the Final Fantasy CG movie for about 9 years. But being a fan of Final Fantasy XIII and Final Fantasy changed that.

I know Final Fantasy XIII-2 has much more better gameplay than Final Fantasy XIII. But if Square Enix didn't create Final Fantasy XIII there won't be Final Fantasy XIII-2 and Lightning Returns Final Fantasy XIII.

But this is my opinion