FFVII over FFVI because of sheer scope. I still can't believe people think FFIX is in any way close to FFVI or FFVII, but I have to respect that at least.
FFVI has an awesome story, one of the best of its generation.
FFVII is still perhaps the most epic game ever and a great story to boot.
It's interested that in both games they have a beginning part where you build the story up and a second end part after a cataclysm to the end of times.