Gold Crown

For a long stretch of the game's life this was pure decoration. Now the reason most people smelt one is the Slime Crown, the item that lets you pick when and where to fight King Slime rather than hoping he turns up or sitting through an entire Slime Rain.

Making it turns your gold into a consumable, so decide up front whether you want the hat or the boss. If you want both, make two. The platinum version does the same job, so the ore your world rolled changes nothing here.