Is Jean Grey/Phoenix really the most powerful of all mutants?
Well in answer to all questions, yes Jean is extremely powerful in her own right, Phoenix or not. Jean is an Omega-Level mutant as previously stated “The Final Mutation”. However she is not the only Omega, counted amongst these are the likes of Iceman, Franklin Richards, Vulcan, Rachel Summers, and Elixir (with other still to be identified). The Omega potential was seeded on Earth a very long time ago, with the intention that they would eventual evolve and rise to replace the Abstracts (Infinity, Eternity, etc); and as such renew the life cycle. So in short, yes Jean is very powerful, but she will be joined by others of equal (if not varied nature) to be come the new Abstracts within the Marvel universe.
although the phoenix has the power of a god in this dimension there is a more powerful mutant aka Franklin Richards the son of Reed and Sue Richards of the fantastic four he is listed as a omega level mutant the same level as the phoenix/Jean Grey. An example of his power was in the fight against onslaught in the comics “To defeat Onslaught, the Fantastic Four, the Avengers, the X-Men, and several other heroes destroy first his physical form, and then his psychic form. In the process, Franklin’s parents seemingly die. It was at this time that Franklin displayed his true power, single-handedly creating the “Heroes Reborn” pocket universe to contain the heroes who had “died” in that adventure”. He is also able to do such things as become Galactus simply by willing it, and all this still as a child before gaining the full extent of his abilities at puberty. Lastly the reason he is stronger then then the phoenix specifically is because “the adult Franklin Richards seen in the present of Ea
