I'm going to say IX. Although Kuja was powerful enough to have just killed everyone from the beginning anyway. I mean he didn't really need the eidolons, as even Bahamut only left him with a scratch. And it seems that only the party could have stopped him, so if he had just killed them back when they were weak he would have been able to do whatever. Riding his Nova Dragon in his girly clothes and setting black mages on villages.