My favorite would have to be the Zora Mask. For one, the transformation looks pretty good. Along with this is the moveset. For basic attack, Link has Zora kung-fu action. It looks cool, but it's not the most practical. Then we have the boomerang fins, which are very helpful in many situations. Then we come to the magic-draining electrical shield, a very useful manuver as well as an over-powered one with the use of the Chateau Romani. And then we come to the feature of show, the swimming. In this form, Link can swim at a high speed with smooth controll as well as drop down to the bottom of the ocean floor or wherever he is, akin to the Iron Boots in Ocarina of Time. He can even use the electric shield while swimming, a very useful attack. All in all, a fun and memorable transformation.
A runner-up is the Mask of Truth. Speaking to the Gossip Stones to learn some backstory as well as some random trivia was a fun concept and something I'd enjoy seeing more of in future games.