Spoilers it is not The Phoenix.
Let me clarify a couple of things about that statement. Jean Grey aka Marvel Girl/ The Phoenix is a powerful mutant. She hosts both telepathy and telekinesis. Even having received training from Professor X on how to use her telepathy. The Professor being regarded as the most powerful telepath in the world.
With all that being said, the ungodly power is not from her own mutant power, but the Phoenix force. Which is a cosmic entity that is supposed to be the immortal representation of life. Meaning it is responsible for giving life to the universe. Not creating it, but giving it life. An odd distinction I know.
Even if you include the Phoenix Force into her powerset and say “Jack you are splitting hairs.” She is still not the most powerful. She pales next to whom Marvel has officially declared the most powerful. More spoilers, I am going to mention a couple of more OP mutants along the way.
Did you know there is a 3rd Summers brother? In a convoluted retcon, he was introduced a while back. This came after two decades of speculation because of one misspoken line from Mr. Sinister (Apocalypse’s Assistant). You probably know about Havok, Alex Summers. We all know and sometimes love Cyclops, Scott Summers. They have another one that is far more powerful.
His name is Vulcan, Gabrial Summers. Havok and Cyclops cannot fight each other because they absorb each others form of energy attack. Vulcan does that to all energy that is presented to him. So if you use any type of energy attack you just made him more powerful. The only exception seems to be that he doesn’t absorb kinetic attacks. So punch away, if you can.
He is reserved for fighting cosmic enemies in a galaxy far far away. Most of the time. Only introducing himself to the X-Men to reveal that Professor X erased their memories of him. (BTW the Professor has had a lot of skeletons let out of his closet recently.) He stomped the current X-Men, then dipped to basically work where the Guardians of the Galaxy operates.
With the power to absorb all energy and attack you with it, he is still not the most powerful.
Going to keep this one short, it is not Apocalypse ether. Although most of his power set comes from alien tech, even with that, he is not the most powerful.
Storm and Iceman have both been classed on the Omega level. Their powers reaching world destruction levels if they were so inclined. You also have Magneto or his younger clone. You have Cable and his alternate reality variant X-man. For good measure I am going to mention Wolverine… for no reason.
Did you know that Sue Storm-Richards and Reed Richards have a son? His name is Franklin Richards. His older version with his powers was brought into the present from the future. Wanting to help his teammate Rachel Summers (Alternate daughter of Jean Grey and Scott Summers). Except he had died in his own timeline… and decided to not be dead anymore.
Some of you may be thinking ‘how?’. The answer is that his power is literally just described as reality warping. You know that thing that Thanos needed an Infinity Stone to do. Except he can do it on a much larger scale and at will. He can make reality what he wants it to be. Something that had been reserved for Cosmic/Eternal beings.
Now I mentioned that Marvel officially declared him the most powerful mutant. It was done through the Herald of Marvel (playing on his troupe here) Galactus. As the universe was approaching the big crunch (the opposite of the big bang) he is dying of old age. Franklin Richards, the child of the team who beat him often, is comforting him.
Galactus basically said that Franklin had the ability to design and create the new universe according to his will. So basically calling him a god. However, take a step back from your screams of blasphemy. The remaking of the universe is more metaphorical as Marvel is in the process of rebuilding their whole comic book universe. Their multiverse and everything becoming convoluted with years of rewrites and different writers. They are trying to simplify it.
So the first child of one of the oldest superhero teams, or at least the longest surviving, is being given the task of recreating the universe. That is the symbolic nature of that conversation. Not to mention setting to rest the nature of Franklin Richards power scale.
Also going to point out we have a Fantastic Four movie possibly in the works. Now that Marvel has taken it back. Will it just be another origin story or like Spiderman, will it be glossed over. I hope for the second one.
If you want another character broken down or some explained, leave a comment.