Best replay value

I only put Final Fantasy I-X on this list on purpose.

Well I'm currently playing through FFVI for the 7th time, so yeah... FFVI. I played through FFIV twice, FFVII twice, FFIX was too boring to finish once, and FFX was too boring to play for more than an hour.

I have only played VII,IX, X and currently playing VI, but I think FFX has a fair bit of replay value.
For example, customising different weapons, changing people's direction around the sphere grid. Then there is maxing out your strength and health, trying to defeat all the monster arena bosses, defeating dark aeons (in PAL version).

So maybe I wouldn't replay the whole story a lot, but for just turning it on and having a save near the end where you can work on defeating dark aeons, etc, I think it has replay value in that sense.

FFI wins, because of all the different combinations. You can only have so many combinations of Espers or Materia or whatever. Since you start from scratch with 4 characters who are going to be the same class all the way through, it completely changes how you go through the game. And that's not even counting parties of less than 4 and/or skipping class change.