Garland --> Abducted a Princess and ascended to Chaos
Emperor --> Brought suffering to the whole world and became Undead and Summoned Pandemonium
Zande --> Seal the crystals and summoned Dark Cloud to engulf the world in darkness
Zeromus --> Manipulated Kain and Golbez
Ex-Death --> Combined 2 worlds and destroyed tons of villages
Kefka --> Rearranged the face of the planet!!!
Sephiroth --> Killed Aeris, summoned Meteor
Ultimecia --> Compressed Time
Kuja --> Destroyed Terra
Seymour --> Absolutely Nothing (SOMEHOW managed to kill the Ronso Tribe.)
MQ and Tactics are unknown to me.
I vote Ultimecia
Note: Glitches have been edited and Zeromus info added
