Magneto has been around since the inception of the X-Men, providing them a source of evil to direct their powers against. Though his backstory has been heavily modulated (by different writers over time), the fact remains that he is a Holocaust survivor — an experience which both guides and drives his thoughts & desires to a great extent.
As one of Marvel's most prominent villains, Magneto has long advocated mutant superiority, going so far as to destroy cities and topple governments. However, Charles Xavier strongly insists that he has some good in him, a confidence that is usually betrayed, but has also been proven right on a number of occasions.
10 Best: Leads The New Mutants
After Magneto saves Charles Xavier from dying (due to the Fenris twins' attack of revenge), the latter's occasional lover Lilandra arrives, saying that only Shi'ar medicine can cure him now. Charles then requests that Magneto take over the position of headmaster at the Institute, providing the sense of leadership the New Mutants sorely need.
He grudgingly agrees, but only under the pseudonym of Charles' cousin, Michael Xavier (an identity already invented and kept ready for such a situation.) He actually rises to the task and is well respected by his students. Not for long, though.
9 Worst: Betrays The Mutant Cause
After years and years of going on about Mutant Rights, demanding the basic freedoms and dignities that world governments had reserved for humans, Magneto does an ethical back-flip.
The X-Men shift base to San Francisco in the Manifest Destiny storyline, and a party is thrown for them by the Mayor — which looks like it's going really well in terms of public favor. Here, Magneto decides to activate Sentinels against the X-Men (an act both ironic and hypocritical, given his own history with the mutant-killing machines.)
8 Best: Holds Out Against The Marvel Zombies (For A While)
An infected superhero arrives from a different dimension, converting Marvel's greatest into starving zombies. In three days, they have licked the planet clean, and are already complaining about their "hunger." Magneto is one of the last remaining superheroes, managing to half-behead Captain America, before escaping to an underground tunnel to bide his time.
Unfortunately, before he can contact his allies on Asteroid M, the zombies locate him, consequently ripping him to shreds. Magneto does not go out with a whimper, however, as he decapitates Hawkeye in a final act of resistance.
7 Worst: Tortures Wolverine
Upon hearing of Magneto's intentions, the UN Security Council triggers a bunch of satellites (intended to alter the geomagnetic nexus in way that stalls the Master of Magnetism from using his abilities.) Given the epithet, however, he manages to rip apart the space stations, forcing Professor X to send the gang after him.
Magneto's aggression has reached fever pitch by now, because he responds to the intrusion by plucking the adamantium clean off poor Wolverine's skeleton. The resultant agony is vast enough to enrage Charles, who reacts with equal violence — emptying Magneto's mind and forcing him into a coma.
6 Best: Protects Bird-Brain
In one of the tamer versions of Magneto, specifically during his headmastership of the New Mutants, a few of them bring back a genetic experiment (gone seriously wrong) after one of their missions.
The creature known as Bird-Brain is unable to communicate properly, nor act in a manner that wouldn't expose the mutants, so Magneto considers letting someone else take charge of the beast. However, the students persuade him to relent, which he finally does upon seeing that Bird-Brain is not, in fact, bird-brained.
5 Worst: Gets High On Sublime
After his triumphant resurrection (for the nth time) Magneto takes control over the U.S. East Coast (successfully countering the X-Men.) His powers are far too intense for any mutant to stop him — the secret is Kick, an inhaler-type chemical that magnifies his magnetism.
While intoxicated, he keeps Professor X in suspended animation, and wreaks havoc across the city, forcing most humans into slavery. It takes the combined efforts of Jean and Wolverine to take him down for good, and even then he "kills" Jean by sending a massive electromagnetic pulse through her body.
4 Best: Shields San Francisco From An Earthquake
During an interview with P.R. super-agent, Kate Kildare, she tells Magneto that San Francisco is unlikely to accept him unless he proves himself in an undeniable manner. As luck would have it, a massive earthquake is predicted to hit the city in minutes.
Magneto takes the opportunity to hold every structure in place (by gripping their bases in place and simultaneously manipulating the iron present in the mantle to smooth the tremors.) At the end, he tells her that the people will understand his importance, given that the act itself will be seen as his "press release."
3 Worst: Abuses His Own Children
Magneto has never been nice to his children. Actually, scratch that: He has been downright abusive to both Wanda and Pietro. He constantly mocks his son, calling him a drain on the vision he claims to have for mutantkind. Quicksilver tries to show his loyalty on several occasions, but instead of the expected fatherly love, he receives a barrage of insults.
In the House of M, Magneto coldly murders Pietro for being a part of how events play out. Scarlet Witch is so upset with her father that she reprograms reality, but this time removing the powers from all mutants in existence.
2 Best: Genosha
Although he only obtains the island of Genosha by threatening to switch the magnetic poles of the Earth, he quickly demonstrates that he can be a great leader indeed. At its peak, Genosha consisted of over 16 million mutants who could, for the first time in history, live their lives freely and without fear of being punished or killed for their nature.
The country soon sees its first civil war when the Magistrates revolt against Magneto (with the help of the human inhabitants of the island), but the rebellion is suppressed without much effort.
1 Worst: Genocide
In staunch opposition to Professor X's humanitarian efforts, Magneto has always despised Homo sapiens for their cruelty. As such, he puts his money where his mouth is, destroying people, cities, everything he can get his grubby little hands on. In Russia, he activates a volcano, which then wipes out a little town in Siberia.
Although he allows people to escape, he threatens that the next time he will go directly for Moscow (and that he "will not be so merciful.") In the Ultimatum narrative, Magneto lets loose a tidal wave against NYC, killing millions of people including Beast, Daredevil, and Nightcrawler. For good measure, Magneto proceeds to break Charles Xavier's neck.