When I was a kid I always played MM wearing the Bremen mask, simply because I though it looked cool. Aesthetically though the Couples Mask is the most beautiful imo.
It depends on what you mean by my favorite. As for as actual utility, the Bunny Mask by far was used the most. In fact, I used it so much, it was to my detriment sometimes as I went flying through the air a little too far. As for just aesthetic, the titular Majora's Mask just looks great.
As far as design goes, I would have to say that Fierce Deity's Mask takes the top spot. In terms of usefulness, however, I'd place the Bunny Hood at the very top of the list.
aesthetically it would have to be majoras mask its just so creepy but pretty at the same time. Functionally it would be the bunny hood because it probably gets used the most and the fierce deity mask for sheer firepower and awesomeness.