most powerful FF villain.

[Kefka_Almighty]

were not gonna let this thread die are we i mean what is the only boss you had to destroy piece by piece

sin

Technically, that's not true, as we can say there were two bosses in FF VII you had to destroy piece by piece, seeing as how Sephiroth threw JENOVA beings at you that were created through pieces of her body, thus causing several fights with JENOVA entities, and you split your party into three groups in order to destroy the three parts of Bizarro Sephiroth before fighting Safer Sephiroth.

I'd like to say Kefka was the most powerful, but the more I think about it, the more I think there were really much more powerful villains than him; I've really come to the conclusion he was just the most sadistic and evil.

I'd have to go with Ultemicia, do to the fact that she basically effectively stopped time and could re-write history if she so desired.

Also, no offense to those saying Sin can return, but (SPOILER) Sin could only return because Yu Yevon hadn't been sent to the Far Plane, as well as the fact that the Aeons were still active and being used. Seeing as how they defeated Yu Yevon and allowed the Aeons eternal respite, as well as not having used a Final Aeon (the usual process of defeating Sin, by which we learned Yu Yevon would then take control of and recreate Sin), he can't come back as he has nothing to attach his spirit to and reform Sin. Bahamut had explained that Sin was basically his armor and that the real Sin was the Final Aeon that Yevon would take possession of. Since no Final Aeon was used to defeat Sin this last time, Yevon had nothing to cling to and thus could not come back.