Okay, let's look at everything here. The Empire would fall before the Covenant, and the Rebels would would be destroyed by the UNSC. Precursors and Celestials, from what I've read, basically can both play God, so we'll call it a draw between them. The Halo Rings would nuke everything in the Star Wars universe, and the forerunners would mop up anything left over, with their own powers and the Promethians. And the Flood would provide support.