Firebolt, but that causes two of your perk on your subclass tree needed to be used to make those grenades useful (Ignite and Viking Funeral). With Fusion Grenades, you have Starfire Protocol so you can gain an additional fusion grenade if you're not running Light Beyond Nemesis and also not use Gift of the Sun so you can have Angle of Light, which can not be used but gain an increase to Agility, or continue to use Ignite and either Radiant Will or Sunburst, which can lead to more orbs, plus more supers, thus rinse and repeat.