Halo by far. The forerunners, precursors, the full covenant fleet and also the UNSC's full fleet, the endless amount of promethians and also the flood. All it takes is one flood spore to infect a colony and then there's a massive epidemic. And if worse comes to worse comes to worse we move our left over resources and soldiers to the ark and activate the halo's to wipe out everyone in the radius of the halo rings whilst they fight the the endlessly regenerating promethians and don't expect to get obliterated by the Devine rings.