Yes, I know, he's an old dude who is stuck in a wheelchair and hardly does anything in the movies. But an argument can be made that if he chooses, Professor X can be the scariest mofo in comics.
In my intro, I kinda made a joke about it, but here's my proof:
He's the strongest telepath on the planet. Add that to his genius level intellect and you've got a powerful combo. "But Khaavy, if he's that powerful, why does he even need the XMen? Why doesn't he just handle everything on his own?" you might ask.
One because he's a teacher, not a dictator. The XMen are his students, not his minions. He trains them to run missions so that they can grow up to be strong, morally or otherwise, so that they can be heroes in their own right.
Two is because he's so smart, he also understands that with great powers come great responsibilities. No, not Spiderman's epithany of helping others. Prof's responsibility is to not turn into the biggest tyrant on the planet. Imagine being able to do anything you want and having the will power to not do it. What do you mean, "anything you want?"
Are you guys familiar with the Purple Man from the Jessica Jones storyline? He's a villain who can tell anyone to do anything he wants and have them do it? With that ability, he was able to kidnap Jessica Jones (an Avenger at the time) and turn her into his sex slave for year. Long story short, he messed up her life bad. You can catch the aftermath in her TV show on Netflix if you're interested.
Well, Professor X can do this too, but he can do it from across the planet. He literally can astral travel to your house a thousand miles away, and mentally order you to shoot yourself in the head. Or write some weird ass law if you're a world leader. Do you know why Magneto and the Juggernaut wear buckets on their heads? Those helmets are telepathic shields for fear that Professor X will one day lose his morals and take control. They live in fear of ever taking off their helmets because professor X might be just waiting to take over. What kind of life is that for two of the strongest mutants on the planet?
Not only can he take control of you, he can read your thoughts. Not just the surface stuff. if he wanted to, he could manifest an astral version in your mind and rifle through your lost memories and locked up secrets. Nothing in your head is closed to him. And he could also insert memories, he can manipulate your mind into thinking he's your long lost grandpa and you'd believe it. You'd fight anyone to death because no one is gonna hurt your grandpappy.
See? If it wasn't for his morals, the professor would easily control the planet. But he's only one man, no one is that powerful. Well, here's the thing, he's got at least 50 super powerful mutants who would honestly give their lives to defend him, even if he turns to the dark side. That's pretty cool, right? years of training superheroes pays off. But that's not even a drop in the ocean of the kind of army the professor has access to. I am, of course, referring to the Shi'Ar empire. Professor X knocks boots with their Empress and had previously served as her Consort and Warlord. The Shi'Ar, in case you're not aware, is a galaxy spanning space empire that rivals the Kree and Skrulls. At one point, the Shi'Ar was the strongest of the three empires until internal strife caused it to focus internally on stabilizing itself. But as her consort, if he ever needs it, he could probably ask her to send in a contigent of maybe a few million Shi'Ar supersoldiers to give him a hand?
As the empire's Warlord, the Professor was given a badass set of black battle armor that gave him flight and enhanced strength and durability and the ability to fly in space. But his real weapon was his telepathic ability, which he used to dispatch any and all who would challenge the Empress's rule. If memory serves, no one in the galaxy could beat him in a one on one. A dragonlike alien who supposedly came from a race of telepathic battlers was batted into the ground, whimpering "so powerful, so powerful!" before collapsing.
It bears repeating, during his time as warlord during a pretty chaotic time in a galaxy wide empire, no one from that empire of trillions of trillions could beat him.
I was going to say something about the Shadowking, but after this statement, I really don't think it matters. Anyway, the Shadowking is this being of pure psychic energy who secretly controls a lot of humanity while hidden in the astral plane. hence the name, Shadowking. You know what happened to him when he ran into Professor X? This was when the prof was still young and still learning about his abilities. The professor focused all of his mental energy and lasered his ass out of the astral plane. While still not at full power and not understanding his full capabilities, the professor destroyed an archetypal evil that's plagued humanity since we were cavemen. Sure the Shadowking came back later on, but that's comics for you.
So, super genius, able to know what you're doing before you do it, able to just mind control you, fuck having to counter your moves or schemes, just take over. Has tons of superpowered lackeys as well as a star empire for backup. Anything else? Yes, yes there is.
In an episode of What If, What If Professor X became the Juggernaut. As you know, Bob, the Juggernaut is the professor's brother, who found the Gem of Cyttorak , giving him the powers of the Juggernaut. I like the duality of having the brothers each exemplifying strength and mental abilities. In the What If, instead of the brother, the gem wound up in the professor's hands instead, making him mentally and physically the strongest person on the planet. That issue made a huge deal about how it made the professor more dangerous. But even as a 9 year old kid, I realized that super strength meant nothing when you can command Thor and Ironman to do whatever the hell you want. Juggernaut level strength just mean he didn't need to mind control some bodyguards for himself.
But this issue also demonstrated the professor's greatest feat. What? Mind controlling a god or commanding an alien empire aren't good enough? he can do more? Yes, Bob, he surely can. Are you ready for this? I forgot the exact details, but in order for him to take over the planet, he needed to build a device to a certain specification. But the professor, genius though he might be is not all that good at building sciency stuff. And the device he needed can't even be built by the likes of Reed Richards or Forge, or Tony Stark... what's a guy to do? Mentally link all those guys together into a superbrain, that's what. Yes, the professor was able to network some of the greatest intellects in the universe into something even greater. Ponder that as you try to find a feat that any other hero has done that might top this.