I played Majora's Mask for the first time this year. I have played OOT since it came out in 1998. Before I played MM, I told myself that there was absolutely, positively no way that it could best OoT, which I considered to be the best game ever made. I was wrong.
Majora's Mask is an amazing experience. The story, the music, and the characters all come together in this unique game, which is completely unlike any other Zelda game in history. Althought OoT is probably the most solid traditional Zelda release, MM has something that OoT lacks, and that something is what makes it better, in my opinion. The dark apocalyptic feel to the game simply works, and it grips you from beginning to end. I feel much more immersed in this game than I do OoT. Even after I'm done playing, I can't help but think about the enjoyable quest I just completed. I start thinking about things like "Where is the entrance to Termina in the Lost Woods"? "How does Link get back to Hyrule"? "Did Link pass through to Termina's world when he fell down the hole, or when he went through the twisted tunnel"? The game is just a lot deeper than OoT, and despite having less dungeons, there seems to be a lot more to do.
Sorry OoT, but MM is a bit more special and exciting.