Following The Avengers at # 5 and The Flash at #4, coming in third in our countdown is Marvel’s merry band of mutants, the X-Men. There probably isn’t a more diverse group of villains that those of the X-Men. They have enemies ranging from those capable of destroying entire solar systems to super-powered assassins. This is a rather lengthy list, so I’ll try to be brief with all of them.
Obviously first and foremost among X-Men baddies is the Master of Magnetism, Magneto. Rarely does a villain and hero represent such diametrically opposing ideas as the X-Men and Magneto. For most of his time in comics Magneto has sought to subjugate mankind under mutant rule, while the X-Men fight to be accepted by and live in peace with humans. He even formed his own group of mutants to fight for his cause called the Brotherhood of Mutants. While he is the oldest and most persistent villain to Professor X’s team of mutants, he has on multiple occasions been a part of, and even lead, the X-Men.
Much like Magneto, Mystique has been a major antagonist to the X-Men both as an individual and as the sometimes-leader of the Brotherhood. The Brotherhood’s roster has always been in a constant state of flux, the most prominent members include Toad, Mastermind, The Blob, Pyro, Avalanche, Destiny, Sabretooth, Sauron, and even briefly Magneto’s twin children Quicksilver and Scarlet Witch before they reformed and began fighting for good instead of evil.
Jealous of of his step-brother, Charles Xavier, and what he perceived as preferential treatment from their father, Juggernaut hated Professor X long before he found the Gem of Cyttorak that granted him superhuman strength, near invincibility, and unstoppable momentum while in motion. His only real weakness is to psychic attacks, so he (like Magneto) wears a helmet that keeps telepathic mutants from being able to attack him psionically.
Created by Dr. Bolliver Trask as a means to exterminate the mutant population, Sentinels have long been a major threat to not only the X-Men but all mutants. Their level of threat was never more on display than in the “Days of Future Past” story arc, where it is revealed that in the future, after a US Senator is murdered by a mutant (Mystique), the Sentinels wipe out the vast majority of mutants and other super-powered beings, and the few that are left are rounded up into internment camps. In a desperation move, the future Kitty Pryde is sent back to the present to stop the assassination from taking place.
Bent on creating an army of mutants to fight for him in what he believes to be an inevitable war between humans and mutants, Apocalypse is one of the most dangerous baddies the X-Men face. To create this army, he pits mutants against one another in battles to the death, many times against their will. Those that survive he brainwashes, enhances their abilities, and often mutilates their bodies into better weapons. There is no better example of than when Apocalypse captured Angel, turned his skin a pale blue, ripped his wings off, and replaced them with metallic ones with razor sharp feathers that could be thrown at an enemy. Born in ancient Egypt, Apocalypse is believed to be the very first mutant.
Obsessed with DNA and creating the perfect mutant, Mr. Sinister is one of the stranger big bads of the X-Men. While he is a brilliant physicist and biologist, Sinister is also a serious threat in battle as well. He is essentially immortal and has the ability to control his body down to the molecular level, he can change the consistency, density, strength, and appearance of any part of body, making him virtually indestructible. The one thing that has shown to consistently hurt him are the optic blasts of Cyclops, leading him to focus his obsession for perfection on Scott Summers, his wife Jean Grey, and the future version of their child, Nathan Summers, better known as Cable.
When Jean Grey became bound with the entity of pure psionic power known as the Phoenix Force, her already considerable telepathic/telekinetic abilities were enhanced to virtually limitless levels. The Phoenix Force lay dormant inside Jean until muniplulation from Mastermind and a telepathic battle with the then-evil Emma Frost of the Hellfire Club brought it to the surface, becoming the Dark Phoenix. After nearly killing them and draining a sun to rejuvenate itself (killing all life in the solar system surrounding it), the Dark Phoenix was immediately recognized as one of the most dangerous beings in the universe. There are very few beings in comics that can match the Phoenix Force in terms of sheer power. It can destroy all creation in all dimensions of existence if it so chose. This has put it at odds with the X-Men and other peoples throughout the galaxy. The difference is, the X-Men are concerned with not only stopping the Phoenix but also saving their friend in the process.