Majora's Mask had overall the best soundtrack in the entire series so this is a very tough choice for me... So I'll narrow it down to 5 in no particular order
Astral Observatory - This theme, although very brief was a great piece of music. The piano was great and it was a very relaxing theme. I loved this part of the game and I used to go there as a kid just to listen to the music.
Deku Palace - The deku palace theme was a cheerful theme which had cool percussion and a great tune to it. It's one of the catchiest songs in all of zelda and I'd listen to this any day of the week.
Stone Tower Temple ( both themes ) - Stone tower temple in my opinion had the best music in all of zelda along with the credits from LttP. It was eerie and even creepy but, at the same time it was very catchy and would get stuck in your head all the time!
Milk Bar - Amazing, catchy, fun! The only downside is, in less you just go to the milk bar to listen to this amazing piece the music, you only hear it for about 10-15 minutes of gameplay... maybe less
The Song of Healing - This is another one of the best songs in the legend of zelda... it's creepy tone was amazing, and set the mood for the entire quest! It also gave you a feeling of care for the people you were helping, amazing piece of music and is easily one of my favorite zelda themes.
That's what I think... I could go on with many other themes because I can't really think of any bad song from MM, I like the themes from the swamp, mountains, bay and canyon ( yes I know they're just remixes of eachother ) and I like termina field, the Clock tower theme is great too, Koume and Kotake theme, and the Laboratory music in great bay is amazing... Oh can't forget Great bay temple, and woodfall and ... ... ... You get it XD MM has great music!!