Skull Lantern

Most hanging lights can be put on a switch. This one cannot, so once it is placed it is on, permanently. It also always points the same way no matter which side you place it from, which rules out mirrored pairs on either end of a room. Both quirks matter if you are building something meant to go dark on command or something symmetrical.

You rarely need to craft any. The Dungeon is already full of them, they come down in one hit, and a single lap usually leaves you with more than a build will use. Shimmer will not break one back into parts until Skeletron is dead, so a pre boss trip gets you the lantern and nothing else.