I think VII is the most important game in the series and is the game that was most important in terms of popularizing the FF series, so it leads the way for me, here is my rankings for the ones I played all the way through.
1. VII
2. VIII
3. X
4. VI
5. XII
6. IV
7. IX
(I played X-2, but since it's a game that probably did not have the time and effort put into it as the others did, I will not compare it to the others, as that would be kind of unfair to it)