I said Majora's Mask. I feel that they could have upgraded a few of the themes and added a lot more memorable ones. While there are plenty of great tunes in every Zelda game, MM was the only one that made sense for me. TP's soundtrack was great, in my opinion. I would have put down LoZ/AoL.. ..but at the time they were made, they were probably in the top 10 soundtracks on the NES (to me at least).
The topic was the most disappointing soundtracks. At any rate, my least favorite is that of Phantom Hourglass, as most of the tracks on there were merely recompositions of The Wind Waker's soundtrack.