I think it's interesting that ljkkjlcm9 says that the best Final Fantasies are the last on each system. It makes sense. I'd imagine that the last game on the system might be the best made because the company had time to perfect its craft.
Yet, my friends and I have always felt that the games that have the strongest hype, critical reception, and acclaim are the first games on each system (I, IV, VII, X). That's why I always thought that VI was so special...I've noticed that critics' lists will often cite FFs IV, VI, VII, and sometimes X as the best games. VI is the only one that isn't benefitting from the hype-factor of being on a new system. So, it's just legitimately good.