Everything. In Majora's Mask you feel like you're part of Termina's world. Every character react to your actions, has a personal story and his own personality. All dungeons are awesome, especially Stone Tower Temple and Great Bay Temple, and all the sidequests are unique in their own way. I replay Majora's Mask a lot, and I never get tired of completing it at 100% everytime.
And the OST, also the musics are among the most inspired I ever heard. When there's Last End played, I feel really sad for Termina. And it's just one example, I could continue for hours, like with Cremia's hug. The feelings Majora's Mask give to the player are... Just the best. And that's why for me Majora's Mask is superior to Ocarina of Time.
Now excuse me, but Kafei is waiting for me.