Ooh, such a hard choice. XD If I had to pick one, it would have to be gameplay. Zelda is a series renowned for its flawless gameplay -- Ocarina of Time even invented the targetting system that is now used in nearly every modern game. Camera controls are spectacular and don't get stuck behind things like other games' cameras do. :p
I'm pretty sure that the other two -- music and story -- are just things to go along with the gameplay. (Either that, or the gameplay goes along with the story. It depends sometimes. XD) Music is, to me, a great portion of the Zelda series, but it's still just something to hear in the background. The storyline in Zelda games haven't really been super-extra-mega amazing from what I remember. It's always been the gameplay for me. ;D