Sunfury

Nothing else you can hold at this stage lands as hard on a single connection, and the gap to the runner up is not small. Because it is a flail you can also park it inside a large target and hold the swing, which turns that one big number into steady output. Skeletron and the Wall of Flesh both go down comfortably to it.

Held down it orbits you and grinds anything it touches, which is what you want against something big and stationary. Tapped, it launches out and snaps back, which is for picking off things you would rather not stand next to. Learn to switch between the two mid fight instead of picking one and sticking with it.

Keep both in your inventory and they fire together in opposite directions, covering both sides of you at once. That is excellent for holding a corridor during a Blood Moon. It is not free, though: spinning the pair costs each of them a chunk of their damage, so for a straight race against one boss you are better off leaving the Blue Moon in a chest.

Shadow Chests need a key from the dungeon, and the lock boxes fished out of hell crates need the same key, so either way you are past Skeletron before you can open anything. If you fish hell crates while you are down there mining hellstone, you will usually see one without going looking.