Shadow Key

This is the rare key that is not consumed. One copy opens every Shadow Chest in the world, forever, and the chests down in the Underworld hold the strongest pre-Hardmode weapons in the game. Walking into hell with this in your inventory turns a dangerous biome into a shopping trip, so it is worth going out of your way for before you start the Wall of Flesh.

It hides in the special chests of the Dungeon, which means Skeletron has to die first. If the chests down there refuse to cooperate, fishing works as a second route: the locked boxes that come out of dungeon crates roll the same contents. The Underworld side of the loop runs the other way, since the boxes you pull out of lava crates are sealed behind this exact key, so keeping it means hell crates stay worth opening long after you have cleared the natural chests.

It has a sell price and it looks like clutter. Losing it does not lock you out permanently, but it does mean going back to the Dungeon and hoping, so park it in a chest with the rest of your keys.