Well, a Zelda game wouldn't be a Zelda game without the Princess or Master Sword (save for a few, like Awakening and Majora). Since it's hard for me to choose between Twilight Princess, Majora, and Ocarina, I'll just do all three. So let's go!
Ocarina: the Ocarina of Time, time travel, and really cool Spirit Temple score, and Navi.
Majora: Time travel, and the masks.
Twilight Princess: Midna and Wolf Link.
You take any of these away from me, and it's war! I have 500 strong men awaiting my orders to take down anybody who takes these away.