Fatebringer. It's useful against fallen for because of the arc damage bringing down captains faster. Firefly is great for packs of dregs/vandals. It's great for the same reason against hive. Against Vex, explosive rounds are bugged and cause 33% more overall damage so its awesome there as well. And for arc burn as well. It's also a legendary, so can be used with Ice breaker or Gjallarhorn/thunderlord.