Melthorn (Legendary Dust). The majority of the damage xbow does is physical damage. Dust reduces PRR more than Shadowhorn or Sparkhorn, which means doing more physical damage.
As a nuker, you'll run Ash. That will also increase imbue damage from the xbow because imbues are treated as spell damage. If you have access to spells that make mobs helpless like Mass Hold Monster, you'll do even more damage while mobs are helpless.