Transformation mask only? Hmm, the Fierce Deity Mask always pwns all, but that's kind of cheating. I've always loved the Deku Mask. I'm a pretty big fan of the Deku race to begin with, and I loved the way Link looked in his Deku form. It was also a nice way to be thrown into the MM universe too, with something brand new and different. The bubbles were a nice change of pace, and for whatever reason, I love the sound it makes when he walks.
Non-transformation mask, I always thought that the couples mask was a nice touch. Yeah, it doesn't really do anything, but it acts as a reminder to how hard it was to get the first time I played through the game. I wasted many hours doing the wrong thing trying to get it until I finally figured it out. I also love its design.