I think you can't definitively say which one is the best. Firstly because when you look at them, each FF game is different.

Secondly, a Final Fantasy is only as good as the memories you have with it.

However, you can analyze the individual components of each game and rate them compared to the others. I think VII is better than the older games because each of the individual aspects of RPG's (story, music, battles, interaction) were improved upon or expanded with this one. However, VIII further refined it and IX perfected them. X took it to another level.

IMO it's up to 7-10, although VII may possibly be overshadowed because the later games improved the individual aspects that made it great. But it was the game that started them and it's story still sticks out to the present day as unique and epic.