Michael Myers may face his truly permanent, final, irreversible death in Halloween Ends. Considering fan reaction to David Gordon Green's Halloween and Halloween Kills, there stands good reason to believe Ends will be such a financial success it won't truly be the end. But it should be the end of this iteration of Myers, at least, in terms of his mask.
In Halloween Kills alone, Myers and his mask faced some serious punishment courtesy of an angry neighborhood mob. But that event isn't alone, and Myers' mask is now practically torn to ash and blood-covered shreds. Even still, his chosen face is far from horror history's most grotesque.
Ghostface Mask/Costume - Scream Franchise
The Scream franchise has proven to be a terrifying property, and except for Scream 4, it's been a complete financial success. A big part of this is the genuinely memorable appearance of Ghostface. With that being said, it's still a blank slate mask.
It's intended to look like a dime store costume (particularly in the first film), and this is just one thing that links Scream back to its forbearer, which is John Carpenter's Halloween. Ghostface (no matter who's wearing the get-up) is intimidating, but it's not because he or she is wearing a hideous mask.
The Shape's Mask - Halloween Franchise
Michael Myers' most memorable moments are always intensified by the void that is his "face." The thing that makes Myers' mask chilling is the fact that, after nearly 45 years of franchise-building, it basically is his face. But, in reality, the face is actually William Shatner's (painted white). It was a mask used for the actor's Captain Kirk but, eerily enough, it was for a dead Captain Kirk (via Star Trek).
This (pseudo) natural expressionless nature gives "The Shape" a visage that seems to hover between pulse-filled and flat-lined. The mask has changed a lot over the course of 40-plus years, but David Gordon Green's trilogy is back to the Shatner look. The soullessness is still there, but now it's covered in a fair amount of soot and blood.
Dr. Decker's Button-Eyed Mask - Nightbreed (1990)
Nightbreed, from Hellraiser's Clive Barker, is an underappreciated minor classic filled with hideous creatures. Yet, the one true villain is the motivation of man. No one represents this better than the protagonist, Boone's, therapist, Dr. Decker (director David Cronenberg, who delivers a legitimately excellent performance).
Barker's Decker is as horrifying and repugnant in Nightbreed as it is in the book that inspired it, Cabal. Barker's prose sexualizes the mask in an encapsulating way, and as the director of the film, he does much the same. Decker's mask is nasty because it begs to reveal what's going through the man's mind.
Hannibal Lecter's Mask - The Silence Of The Lambs (1991)
Hannibal Lecter's re-purposed goaltender's mask is disturbing and disgusting by implication. Lecter is essentially known for two things, which is his intelligence and cannibalistic tendencies. The mask's bars over his mouth strongly suggest the very action they're there to prevent.
Lecter is objectively terrifying even if he's behind a reinforced transparent wall. The thought of him out in the wild, walking around with teeth gnashing subtly below his lips and eyes scanning potential prey, is unbearable. The mask lets the viewer know he's out from beyond that reinforced transparent wall, and he's one step closer to being a lion on the prowl.
Sam's Mask - Trick 'r Treat (2007)
Sam, Trick 'r Treat's silent, primary antagonist is a horror villain more likable than the heroes of the film. With that being said, the main characters are far from true heroes, and that's certainly not a title the grotesque little Samhain himself could claim.
Sam appears to be a child in an odd, vague costume tailor (or home) made for the season, with a little orange jumper and a burlap mask with oddly and hauntingly child-like button eyes. However, what's beneath is far more objectively hideous than the indescribably unsettling costume/mask. Sam resembles a little goblin, with orange skin and blood in the form of pumpkin guts. Even still, the mask is gross because the creature is creepily emulating a child.
Pig Mask - Saw Franchise
One of the major horror franchises, Saw, has had several costumes for its killers. However, the most famous (Billy) isn't part of a costume so much as it's part of a giggling little animatronic, baby bike-riding puppet. Of the costumes, the most well-known has to be the pig mask, which has made an appearance in just about every installment of the long-running series.
It's been worn by everyone from Jigsaw himself, John Kramer, to Amanda Young and even the original Saw film's Dr. Lawrence Gordon. With that being said, it's never appeared to actually be a pig's face. It does look like a pig's face, but Kramer and his pupils really just have regular stitched material over their faces.
Jason's Various Hockey Masks - Friday The 13th Franchise
The Friday the 13th franchise has mostly stuck to formula over the years (even in space), but one thing that changes with each installment is Jason's mask. Sometimes it's a slight change due to what happened in the previous installment, e.g. the axe mark from Friday the 13th Part III to Friday the 13th: The Final Chapter. Other times, it gets a complete overhaul.
Most notably, and most disgustingly, is the melted mask from Jason Goes to Hell: The Final Friday. In earlier installments, it's really just a hockey mask. Blood covered, but still a hockey mask. By the time Friday the 13th Part VII: The New Blood rolled around, however, the mask had become absolutely grotesque.
Smiley Mask - Smiley (2012)
Michael Gallagher's Smiley wasn't the scariest horror film of 2012, and even manages to make movies like Slender and FeardotCom look slightly better by comparison. But it did manage to create a scary visual with the title character.
Smiley's mask is flesh-toned, with an equally dermal-skewing texture. The sewing patterns over the eyes and mouth only up the creep factor. Of course, the blank-faced mask isn't like Leatherface's, but it's an icky enough replacement.
Santa's Face - Krampus (2015)
Michael Dougherty's Krampus features a terrifying lead baddie (and many others) meticulously crafted by Weta Workshop. However, no matter how many growling Gingerbread Men or evil Jack-in-the-boxes there are, none hold a candle to Krampus and his mask.
As the anti-Santa, Krampus punishes those who are naughty, leaving Mr. Claus with the nice. But instead of being a kindly, bearded man with a staff of elves, Krampus is a horned beast with toothed minions and a mask that appears to be made of Santa Claus' face. It droops off of Santa in a way that sells the unsettling image, and beyond the physical repulsiveness, it's an insight into the creature's mentality.
Leatherface's Human Skin Mask - The Texas Chainsaw Massacre Franchise
Leatherface, from Tobe Hooper's landmark The Texas Chain Saw Massacre is a no-brainer when it comes to establishing which horror icon has the most grotesque ensemble. It's not leather that comprises Leatherface's "face," but rather the literal skin of his victims. It's a repulsive act, an utter invasion even beyond the grave, and the thought of wearing it is enough to make the viewer's stomach churn.
Leatherface has made his mask out of people's faces regardless of which installment or timeline he's operating in. It's a major part of the franchise and some installments, e.g. The Texas Chainsaw Massacre: The Beginning, go so far as to show Leatherface's mask-making process.