Molten Fury

Feed it plain wooden arrows and it sets them alight on the way out, so the cheapest ammunition in the game arrives with a burn attached. That is the whole reason to use this bow: you can shoot all day without spending anything.

Converted wood picks up the fire effect only, not the extra damage that proper flaming arrows carry. Every other arrow type passes through untouched, so if you already have jester or unholy arrows in the bag, use those and treat the wood trick as your fallback when you run out.

It arrives right at the end of pre-Hardmode, which is exactly when fights start running long. Being able to keep firing through a drawn out fight without watching your ammo count is worth more at that point than a slightly higher number per shot.