Alright folks first of all i am definitely going with my homie KEFKA as the evilest villain. Basically for one reason. He is the acknowledged villain THROUGHOUT the entire game. He burns Figaro, poisons Doma, Discards Espers with wanton disregard for any life force left within them (magitek Facility), Kills Leo, Kills Gestahl, Rearranges the face of the planet and kills multiple cities populations. He then is able to turn himself into a Godlike figure and control the world. Truly an impressive and EVIL villain.

For those of you arguing for that Tree, Give me a break, the whole evil spirit thing is so played out in every RPG that it isn't even funny anymore. Give me a human that becomes completely engulfed by hatred and malice and I will see them as more evil than just some SPIRIT that pops in out of the blue.