Boss Key

The Boss Key is a necessary dungeon item. It actually goes by several names: Boss Key, Big Key, and, in Link's Awakening, the Nightmare Key. It is used to open the boss's lair and other big doors. In A Link to the Past, the Key is also necessary to obtain the item hidden within each dungeon.

This Key has been in every Zelda game to date, and that probably won't ever change.