Jean Grey was the only Class Five mutant ever met by Charles Xavier (which implies Magneto and Pyro are Class Four), and her potential was stated to be limitless.
Magneto was officially deemed an Omega-level mutant, and fans who have seen some of his greatest feats over the years weren't surprised at all.
He is not a mutant so his powers aren't rated on their scale. His regeneration does have its limits, he can grow old, he can die in a cataclysmic event etc.
Jean Grey was a Class Five mutant with immensely powerful and potentially limitless telepathic, empathic, and telekinetic abilities.
Wanda Maximoff (Earth-22795) being described as a Class Five mutant can possibly be explained by the fact What If? Avengers Disassembled #1 (November 2006) was released only a few months after X-Men: The Last Stand (May 2006) who used first that term to describe Phoenix/Jean Grey.
Magneto has been classified as a "Class 4" mutant, being described as greater than Class 3 while Jean Grey is the only Class 5 mutant Charles Xavier knows of.
According to the standard Mutant Power Level Classification of Marvel's Earth-616 (Prime Earth), Wolverine is a Beta-level mutant, which means that he can actually pass off as a human, but only if not observed very carefully. The X-Men are one of Marvel's most famous franchises.
Officially, Namor the Sub-Mariner is considered the first mutant superhero whom Marvel Comics ever published, debuting in 1939.
Strength Level
Class 100+ ; The Juggernaut possesses vast physical strength which exact limit is unknown but he is able to lift far in excess of 100 tons effortlessly. He is one of the strongest beings to ever walk the Earth.
Magneto is a mutant with the power to manipulate magnetic fields to achieve a wide range of effects. He is classified as an Omega-level mutant.
Despite how popular he is, Wolverine isn't an Omega-Level mutant.
Storm has been stated to be an Omega-Level Mutant. Her potential is as of yet unrealized, and on one occasion the Super Giant stated that Storm was an "Omega-Level Mutate", grouping and targeting her with Omega-Level mutants such as Iceman and Rachel Grey. Her Omega-Level status was eventually confirmed.
Although not a mutant, Juggernaut has been featured as a prominent member of the Brotherhood of Mutants. He is also the stepbrother of Professor X.
Thanos is an omega level mutant.
Powers. Quicksilver is an Alpha mutant, allowing him to move at lightning speed. He had super-fast reflexes and his brain actually processed information much faster than most computers.
As the heroes discover soon enough, the future they have been brought to is the site of a near-total genocide against humankind carried out by none other than Hank McCoy/Beast, who now stands as what Quentin describes as the "god of all mutants" in his monstrously mutated form.
Grey, even without the Phoenix Force, is the most powerful mutant in the Marvel universe. Unlike Professor X, her powers go beyond telepathy and extend to telekinetic and psionic powers. Her telepathic, telekinetic, and psionic powers, have virtually no limit.
Now an adult, Loolo Marshall - first introduced as a minor Arakki side character in Al Ewing's X-Men Red - is established as the fastest being still alive, taking on the codename Quick.
PROFESSOR X
Charles Xavier is an Omega-level telepath, but he is only an Alpha level mutant. He has the most powerful brain in all existence and is the world's most powerful telepath.
Alpha Level
He has the ability to transform energy itself. Most commonly, he changes any potential energy in an object into kinetic energy. Gambit can charge up any object with energy, and make it explode with exponential force. That mutation puts him in the second highest power ranking among mutants: Alpha Level.
Powers. Known Powers: Following the genetic alterations done by Apocalypse, Sinister possesses a wide array of powers. Over the years, Sinister has used the genetic material of other mutants to grant himself further a vast array of powers. He has been stated to be an Alpha-Level Mutant.
It might be surprising for some fans to learn Professor Charles Xavier isn't an omega-level mutant. However, he's still an incredibly powerful alpha-level telepath.
Jean Grey wins. Magneto is more experienced, more skilled, more trained, more hardened, and more effective against his opponent than Jean Grey (without the Phoenix Force) could ever be. And thus, he could kill her. Undoubtedly, Magneto could overpower Jean Grey without the Phoenix Force.