Magneto could easily be considered the X-Men's archenemy and is one of Marvel's most enduring villains.
What puts the Master of Magnetism head and shoulders above most other super-criminals is how complex his character is. As a Holocaust survivor, Erik Lehnsherr has seen the worst of what can happen when one race considers themselves superior to another. Because of this, Magneto fights to make sure that mutants are never subjugated by mankind using whatever means necessary.
This noble motivation has seen Magneto earn a measure of respect from the X-Men and he has even joined their side on numerous occasions. However, Magneto is still a supervillain who isn't afraid to get blood on his hands if it gets the job done. His extremist views on how to best protect mutantkind have led to him committing some pretty heinous acts over the years.
While some of these could be justified as necessary evils for the greater good, others demonstrate an inherent hypocrisy in Magneto's actions. He is often willing to inflict the same abuses on humans that he fights so hard to protect mutants from.
And then there are the times he just straight up tries to wipe out all life on Earth.
10. Formed The Brotherhood Of Evil Mutants
No supervillain is complete without a team of loyal minions to back them up.
In Magneto's earliest appearances, he lacked a lot of his more nuanced character traits and was more of a traditional supervillain. Still, referring to your lackeys as "Evil Mutants" is a little bit on the nose.
The original Brotherhood consisted of the super-agile Toad, the illusion (not trick) creating Mastermind and unwilling members Quicksilver and Scarlet Witch, who had no idea that they were really Magneto's children (and now aren't because comic book continuity).
In their first appearance, the Brotherhood helped Magneto take over the country of San Marco by using Mastermind's abilities to disguise themselves as an invading army. They were of course stopped by the X-Men, but went on to help Magneto with various over evil schemes, like kidnapping Angel and sending Toad to infiltrate the X-Men.
They were also joined by new members, like the teleporting Vanisher, forcefield generating Unus the Untouchable and Blob, the amalgimation of ever "before" photo ever taken.
Eventually Scarlet Witch and Quicksilver rebelled against their guy-who-used-to-be-their-Dad-but-isn't-anymore and joined the Avengers. Soon after, the Brotherhood of Evil Mutants disbanded. Magneto went on to form a second team, but they all had the same mutant power of being impossible to care about.