I really do have to question: Is FFX able to be classified with 'Classic' Final Fantasy? I mean if so then X-2 is certainly also 'classic'.
I think FFX has far more in common with later entries like XIII than with the PS1/Snes/NES entries. I just can't really think of a single metric that would dictate FF-X being some sort of cut off point. The only slight possible one being that it was the last worldwide FF game to be published by Square. However FFX-2 was developed before Square-enix and published in Japan before square-enix.
So what exactly makes FFX the cut off point? Seems to me that 9 is the more logical choice. As after that you lost the traditional world map, voice acting came in etc etc.
So yea gotta say for me it's 1-9 is classic. 10 and later is new style FF.