Flood.
I don't care how powerful the Magic Schoolbus is. The Flood has Camnator. He could easily reflect any attack thrown at him with "put that mirror down" or "stop talking about yourself." And that's not even considering his special "you've gone incoherent" and "botched lobotomy" attacks or his ability to clone himself with limitless alts or regenerate from death with the power of weedposting and booty spam.