Legion, the son of Professor X, is more powerful than all of the X-Men combined. When it comes to super-powers, skill is as important as power. For all that's the case though, the most powerful mutants - so-called "Omega mutants" - outstrip all their fellows. That term has been tossed around for decades, but writer Jonathan Hickman finally defined it in 2019, revealing an Omega mutant is someone whose powers have no upper limit.
Omega mutants include characters like Iceman and Storm, Magneto and Jean Grey. But the greatest of them all is certainly Legion, the son of Charles Xavier. Disturbingly, Legion was born because of Xavier's manipulations; Professor X wanted a child with the power to manipulate reality itself, and geneticist Moira MacTaggert helped him find a woman whose genes were compatible with Xavier's, and would potentially lead to the birth of a reality-manipulator. Xavier maneuvered himself into the life of one Gabrielle Haller, a traumatized woman who he supported as a counselor, and the two had a sexual relationship. When they parted ways, Gabrielle was pregnant with a son, whose existence she concealed from Professor X for years.
David Haller's mother became Israel's ambassador to the United Kingdom, and one day a trip to Paris for mother and child went tragically wrong. They were caught up in a terrorist attack, and David's fear triggered his mutant powers. He killed the terrorists, but as he did so, his powers connected with them telepathically absorbing their minds into his own. That was the day David Haller became known as Legion; he has absorbed countless other minds over the years, while his brain has gone on to create other personas. Each one of David's personalities has access to a different powerset, and, incredibly, David has claimed over 200 of his personalities are Omegas.
Legion's sheer power was best demonstrated when Magik brought him into play against the Elder Gods. These monstrous beings had ruled Earth in prehistoric times, and had been banished to another plane of existence; they had long sought to return, but Legion stood as Earth's greatest defender. He faced a threat all Earth's sorcerers combined could not have defeated, and he erased these Elder Gods from reality with apparent ease. It's no exaggeration to say Legion may well be the most powerful mutant ever born - certainly now Franklin Richards is no longer counted as a mutant.
And now he's on the mutant nation of Krakoa. The modern mutants consider themselves gods among men, but if that is the case then - when compared to Legion - they are like everyday Asgardians standing in the presence of Odin the All-Father. The truth is that Legion has more raw power than all the X-Men combined, and indeed possibly than all the mutants alive combined as well. He's not to be underestimated.