Would have to be Twilight Princess, I think this soundtrack is definitely one of the most underrated when it comes to this kind of discussion. Honestly when it comes to soundtrack there are 2 distinct games that come to mind and they are Majora's Mask and Twilight Princess, for some reason I think it's the contrast that I love about these two.
Twilight Princess is definitely a darker Zelda game than most and it had more realistic situations and themes, which in my opinion worked hand in hand with the soundtrack. An example would be the theme of the Twilight Realm, it was dark and mysterious and in a way fearful, this suited the current situation in my opinion as we saw the terror that had took over Hyrule. Midna's Lament would be another example, this theme did have this sort of magical tone but it was quite depressing which again suited the current situation, Midna was on the verge of death, this track really captured the moment and made it feel more realistic. I think there were quite a few fresh additions that set a dark and mysterious tone to this game and in that way it kind of stood out to me.
There were also plenty of other track which in my opinion complimented that specific moment or even the setting. Snowpeak would be one of my favourite examples, the soundtrack felt mysterious, it was very quite and mellow which gave the feeling of isolation and this was what Snowpeak really was meant to be. Other themes such as the Faron Woods felt quite magical and suited the actual surroundings as it felt quite natural suiting the nature on the forest. The theme in Lake Hylia and Zora's Domain sounded magical which really complimented the amazing setting and the water effects. Truth be told I could go on a lot of how I thought the soundtrack really worked in the favour of the overworld.
There were also some great dungeon themes, some of the best in the series, plus due to the sheer amount of dungeons in this game it gave a great variety; from the mining sound in the Goron Mines to the ghostly theme in Arbiter's Grounds, to the puzzling theme in the Temple of Time and so on... I would also like to mention an example of how inspiring the soundtrack was, this would be the theme of the main area of the game, Hyrule Field. It actually was very motivational and it made the whole adventure feel epic, like when you entered Hyrule Field you had this incentive or a motive to actually go out and fix what was wrong with Hyrule, this is just one of the examples but I though TP did a great job in this aspect.
I really think that Twilight Princess had a large soundtrack with many memorable pieces that were not only unique and fresh but also each track felt different, it was definitely one of the more diverse soundtracks in a Zelda game to date. Of course you had all the happy/magical tracks but you also had quite dark mysterious tracks combined with motivational and emotional tracks that made very situation more realistic. In my opinion I think a soundtrack should really compliment the game in every way, it should make the player feel what essentially is meant to be felt in that current situation and this was the case with Twilight Princess in my opinion.