I have a hard time actually ranking them - in a lot of cases, I liked each for completely different reasons. Therefore, I'll catagorize.

Best Story:
1. FF7
2. FF6
3. FFX

Self-explanatory.

Best Characters:
1. FF6
2. FF7
3. FF4

Self-explanatory.

Best Gameplay:
1. FF5
2. FF6
3. FFX-2

I like customising my characters, and these three games had the best battle systems IMHO. FF5's job system is the original and the best as far as character strategy goes. FF6 was one of the few games where summons were both an important part of the story and didn't win the game for you, and FFX-2 had really neat ATB battles, with chain attacks and the return of the job system.

Best Aesthetic Value (Music, Graphics, Character and Scene Design)
1. FFX
2. FF8
3. FF9

Nobody has to say that FFX's graphics were amazing, but the design of Spira and the added distinction of voices gets it the top spot. FF8's look blew me away when I first played it on the old PSX, and I still think it has the best music of the entire series. FF9 has some of the best character designs; although their cartoony nature turned me off at first, Vivi's adorable motions and Steiner's clinking armor and temper tantrums won me over very quickly.