Majora's Mask is a favourite no doubt, it was a darker type of world, it was a nice step out from Zelda's EVERYTHING IS FANTASTIC world. It also puts you into a more realistic hero position, in OOT you took your time and nothing would happen, if you sat around MM you saw the moon was going to kill everyone. More incentive.
I loved Majora's Mask a lot, it was very emotional for me and at it's best when I went for the couple's mask.