There are simply too many characters in all the FF games for it to be made into a film. If anyone tried to make a 30+ hour game into a 2 hour movie, may god have mercy on his/her soul because it'll probably end up like Cruel Intentions 2.

However, since this is abstractly speaking. Out of the list you've given I think FFIX would make the best movie. The medieval setting, vast amount of strange races/species, a likeable main character, heaps of special magic effects, comic relief...

There's no better game that could represent 'Final Fantasy' out in the big screen world.