Moon Stone

Everything it gives you is dead weight in daylight and fully active after dark, so it is an accessory you swap in for a job rather than one you leave equipped. That job is the mechanical bosses, all of which are night fights, plus the general night grind where the nastiest surface enemies appear anyway. It also stays live through a Solar Eclipse, which is technically daytime but counts here.

For most classes the mix of damage, crit and defense beats an Avenger Emblem on average output while handing you regeneration and mining speed for free. Summoners give up a sliver, since crit does nothing for minions, but they get survivability and extra minion knockback in exchange. What it will not do is stack with the Celestial Stone or Celestial Shell, which is where it ends up anyway.

Vampires only appear during a Solar Eclipse, and the event has no fixed spawn rate, so a Water Candle and a Battle Potion do more for your odds than patience does. In a Vampirism world you can skip the fight entirely and find it sitting in ordinary underground chests.