Unsurprisingly, there are a lot of different kinds of masks in The Legend of Zelda: Majora's Mask. Some of them are required for beating the main story, some of them will help Link collect a Heart Piece, and some are only usable in very specific situations. Players will need to collect all twenty-three of them, however, if they want to unlock the final mask in the game.
Collecting all of the masks and filling out The Bombers Notebook is one of The Legend of Zelda: Majora's Mask's biggest draws. Thanks to the three day time limit and an infinite amount of retries, players can attempt certain trails over and over again if they miss out on them the first time, something which is likely to happen since some of the masks listed below are rather difficult to get, especially for new players.
There are two different categories of masks in Majora's Mask. The first are the masks which Link uses to transform into different characters, and the second are the masks Link simply wears on his face while still remaining himself. Being that the transformation masks are required multiple times throughout both The Legend of Zelda: Majora's Mask's story and side missions, most will be listed last. In descending order, the most (and least useful) masks in Majora's Mask are as follows:
Majora's Mask - Kafei's Mask
While the quest involving Anju and Kafei may be one of the best mission strings The Legend of Zelda has ever put out, the Kafei Mask itself is hardly necessary. Madame Aroma gives Link this mask in order for him to help find her son, and all players need to do with it is talk to Anju while wearing it to begin their story. Other than that (and a few bits of different dialog from NPCs and one other time Link puts it on during this quest line to appease the Curiosity Shoppe owner) the Kafei Mask is basically worthless and is never needed again.
Majora's Mask - The Giant's Mask
The Giant's Mask may be very useful in the fight against Twinmold - but that's it. Not exactly a transformation mask, The Giant's Mask makes Link into a huge version of himself in order to easily be able to attack Twinmold as it dives in and out of the sand. Unfortunately using the mask continuously drains magic power, and it can't be used in any other part of the game. It's not even entirely necessary, as Twinmold can be defeated without using the Giant's Mask at all if the player wishes.
Majora's Mask - The Fierce Deity's Mask
Inarguably the most difficult mask in the game to obtain (since players need to get all twenty-three other masks first) the Fierce Deity's Mask in Majora's Mask is actually almost identical to the Giant's Mask - it makes the fight where it's usable much, much easier, to the point of making the gameplay worse because the challenge is gone.
The Fierce Deity's Mask can only be used in boss battles, but it makes everything so easy it's almost laughable, and players can't even try out its massive power in the open world.
Majora's Mask - The Garo Mask
Another one-use mask, The Garo Mask will convince the Garo watchman to create a hookshot target so Link can advance into Ikana Canyon - and that's really about the only time players have to use it. If they want to fight and talk to Garo warriors they can wear this mask inside the canyon, but it's not necessary any more for progression.
Majora's Mask - The Circus Leader's Mask
This mask can be worn during the mission where Link must help escort the Cremia Milk Wagon to keep the Gorman Brothers from attacking, but that's really its only usefulness. Since Link needs access to the Milk Bar Latte as well as all three transformation masks, by the time players get this mask they likely won't need to use it.
Majora's Mask - The Mask of Truth
An iconic mask in The Legend of Zelda series, The Mask of Truth allows Link to read the thoughts of dogs and talk to Gossip Stones. Unfortunately, neither the game's dogs nor its rocks have anything especially secretive to reveal.
Neither function of The Legend of Zelda: Majora's Mask Mask of Truth is overwhelmingly important in the overall main game, but Link can use this ability to cheat at the dog track and find out which dog has the best mindset for victory, so at least that's something.
Majora's Mask - Don Gero's Mask
Don Gero's Mask allows Link to talk to frogs. If players can both defeat Goht and talk to all five frogs in The Legend of Zelda: Majora's Mask in the same three day cycle, they will all assemble at the Mountain Village and Link will receive a Heart Piece.
Majora's Mask - The Keaton Mask
Majora's Mask's Keaton Mask is also used for a Heart Piece. Players must attack the dancing grass in North Clock Town road (or one of the other dancing grass areas) while wearing it, and then they have to answer five questions correctly.
Majora's Mask - The Couple's Mask
The ultimate prize for completing Anju and Kafei's story in Majora's Mask, the Couple's Mask is a pretty disappointing gift. While a beautiful symbol of their love, The Couple's Mask doesn't add much to gameplay (and was incredibly hard to obtain) but at least Link can use it to get yet another Heart Piece by talking to the Mayor while wearing it.
Majora's Mask - The Postman's Hat
Also found during Anju and Kafei's story, the Postman's Hat can be obtained by breaking off from the main side quest and giving Madame Aroma's letter to the Postman instead.
Wearing The Postman's hat allows Link to look inside of any mailbox in Majora's Mask, but only the first will contain a Heart Piece. Every other mailbox after the first only contains one rupee.