Any version of Zelda's Lullaby. The reason is because whenever I hear any of those six notes, it brings back such nastalgia. It also lets me know that Nintendo is keeping in mind that Zelda is a connected series. So whenever Zelda gets her theme song, I feel as though the other Zelda's are related somehow.
It is also a very soothing melody. Before hearing this for the first time in OoT, you usually get these either creepy or upbeat tones. Then you walk into the garden where Zelda is spying and it's like the atmosphere changes. It does a great job of being a theme song for Zelda because I think that is the atmosphere her presence is suppose to present.