I just can't decide. Though I would probably pick FFVI. It was very amazing, it introduced pretty much what would be all the story staples in later FFs. It also pushed the graphics of the SNES to the limit, at least for their games. (They usually do with the last game of the console.) It's story was pretty revolutionary, once again, introducing some staples of later FFs. And, well...Kefka is the best FF villian ever. Think you can come up with someone eviller and crazier than Kefka? (Is "eviller" even a word?) I think not.
And the reason FFVII gets so many votes is because it's well...just very popular. It was the first FF released worldwide. Naturally that would mean it was many peoples' first FF experience. FFVIII was amazingly popular too (from what I understand. )
But anyway, I guess I'll just pick FFVI. I love them all though...
Edit: Okay, I guess my vote didn't really matter