Golly gee, that is a difficult question. My first choice is probably Skyward Sword because honestly the sound quality and feel of the soundtrack in general is breathtaking in my opinion. The songs just fit so well with the game. I don't know how else to explain it. But on the other hand, Ocarina of Time has some of my favorite Zelda songs; same goes for Majora's Mask. Crud, but the more I think the more conflicted I feel. Gonna stick with those three as my top choices, but I think the Wind Waker and Twilight Princess' soundtracks are noteworthy as well.