Majora's Mask for me, of course!

The usage of both transformation and non-transformation masks, the sidequests (Anju and Kafei, anyone?), the dungeons (Stone Tower Temple), the places (Ikana and Great Bay), Fierce Deity Mask and just so many other things make me choose it over all the other Zelda games. Not to mention that, after reading that one article (The Message of Majora's Mask, I think) about MM really made me appreciate it that much more. Sure, the article might have pushed it in some points, but I can't deny that MM was a beautiful game for me to experience.

MM is just one of those few Zelda games that I can play through again and again and never get tired of it.