Not to mention what, the last time you guys had someone end the world was in Saikano?
True, FF6 is a old game, but Kefka destroyed it and pulled an AM, kept some people alive and allowed them to repopulate long enough for him to crush half of them to have some left over.
Speaking of AM, as obscure as I Have No Mouth and I Must Scream is, he needs to be on there. Dude killed off humanity, left Earth as a nuclear wasteland that no life could survive on very long, and spent the rest of his time torturing the fuck out of the last six humans alive.
He could've given Giygas a run for her/his/it's money.