Mine has to be the Bombos Medallion in A Link to the Past and the Magic Rod in Link's Awakening. Bombos literally nuked everything in the screen, and is the only medallion that is optional, unlike Ether and Quake, which were used to open Misery Mire and Turtle Rock respectively. The Magic Rod in Link's Awakening just kills things. It literally causes enemies to burst into flames and then die. Only few enemies were immune to it, but otherwise, it would kill off even the enemies that couldn't be killed by other weapons. It also gets bonus points for being able to actually kill a cucco with no negative repercussions.