this is tough. I think the fallen would choose not to get involved and would never be able to be completely wiped out because they are cowards and run away, and they are also the weakest (i mean edz dregs are kinda just big bugs). The cabal have the power to win and military might, but i feel like their species isnt that special and they dont have any good advantages besides their military, especially compared to the hives connection with the darkness and the vex with their time traveling and reality warping. That leaves us with the hive and vex, which is tough. I think overall the hive are stronger, but the vex have a single concious that is connected to all of them through vex milk. They are also very cunning and want to transform the universe for themselves, becoming gods. I think the hive would win becasue of the darkness and intelligence, but i think the vex would just time travel or escape to another universe before they are defeated.