I have to agree with Roto for the moment.
VI and IV can be argued a little..... but not much from my perspective.
And /shrug... IX seems to increase its lead over VII day by day in my mind.

/shrugs...
As for I, II, III, I only really have experience with I at the moment.

And no matter what we say or do.. the majority of people will like FFVII and FFX the most.... although I can not see why. I'd rather have XII and IX.