I've always found the FD mask rather disappointing because you can't use it outside boss battles. I like the bunny hood, in fact i love it and I never play the game without it. Though technically its not a mask, but well... a hood. My other favorite one would be the Bremen mask, sure it doesn't have any combat ability and it only grants you access to the milk bar, but it also grants you access to the ultimate milk that gives you infinite magic for three days! And it also looks cute
From the three main masks, I'm still not sure which to pick up, the Goron or the Zora mask, though I love to swim in the Zora mask more than roll in the Goron mask, and those boomerangs are just cool.