The Flood counteracts Jedi/Sith, The Precursors counteract the Celestials, Halos counteract all Star Wars super weapons, Forerunners counteract the Republic+pre-Republic societies, Ancient Humans counteract Sith Empires, Covenant counteracts the Empire, Spartans counteract the Mandalorians. Can go either way really if one side plays there cards just right. [spoiler]I'm actually just torn between Revan/HK and Chief/Cortana. Why am I such a meatbag....[/spoiler]