I'm gonna try and enter my two cents in on this thread.

In my opinion, Necron--and I am not saying this to undermine, or to incur wrath, but merely to shead light--is not the most powerful. Yes he did still exist in some way after the battle, however in what way he existed is up to debate. His ability to "destroy all existance" does not exist within himself but in destroying the crystal. Yes, he was convinced that beings have the right to exist, but ONLY because of his defeat. Now if he were truly all-powerful and hell-bent on ending all, then he would have had NO problem vanquishig Zidane's party in the end, especially if he were all-powerful.

In my opinion, the most powerful villain in the series to date was Yu Yevon. I say this despising all that is (X) and (X-2). My reason is simply this, Yevon, having ascended past flesh, now could possess pretty much any deitic character or unsent. And as he was merely sent to the Farplane in his defeat, like all other unsents, he could still return...if he still no longer possessed humanity. It is even possible that if Yu Yevon and Necron were to fight, Yevon could possess Necron making him the new SIN.

however this is merely my own opinion.