The hive are the only race tied directly to the darkness. The Vex seem to be unable to compute the darkness, as evidenced by Sol Divisive grimoire: "If the vex found worship and devotion more effective than any other behaviour, they would adopt worship. Whatever the vex found - or made - in the Garden, it transcends even their power", The Cabal just seem to be doing a Roman empire across the universe, and the Fallen are basically a bunch of refugees trying to get their traveller back. Based on raw power and ability I'd say the vex are the most powerful race