Warning! Spoilers ahead for Immortal X-Men #4!
Marvel is once again hinting that Jean Grey is the X-Men's most powerful psychic. In a private moment at the beginning of Immortal X-Men #4 by Kieron Gillen and Michael Bandini, Emma Frost admits why she believes Jean surpasses her, while raising questions about what makes a great telepath.
Emma narrates the entirety of Immortal X-Men #4 from her bedroom, where she's taken on her indestructible diamond form—in which she famously cannot attack or be attacked telepathically—because "a psychic is most vulnerable when dreaming." This new habit also causes her to consider how she uses her mental gifts to begin with and whether she can ever access her full potential.
The majority of Emma's training and application is in mental manipulation. This is how she plied her trade prior to joining the X-Men; it was the trademark of her longstanding position in the Hellfire Club and the skill Kingpin once hired her for. A perfect extension of her own superiority complex, Emma learned how to weaponize the deepest fears and insecurities within her victims to steer them towards a desired outcome. But while Emma's methods only requires her to view people as puppets, she believes Jean's greater well of empathy ultimately makes her the more potent psychic. While Emma prides herself on being untouchable, she suspects it is exactly that quality which prevents her from achieving oneness with another mind.
Emma later makes reference to Xavier as well, wondering to herself how he can deal with the cacophonous influx of emotion from every mind on Earth. But she and Jean have a far rockier history considering the romance between Emma and Cyclops. Though they seem to have buried the hatchet, at least for now, Emma is known to hold grudges. Seeing her confess that she isn't quite on Jean's level—even to herself—is a sign of her vastly increased respect.
Overall Marvel is still figuring out just how powerful Jean Grey is anyway. She recently held Cyclops' optic blasts at bay with her telekinesis, a feat that she previously could only accomplish when souped-up by the Phoenix Force. When Jean was young and still going by the name "Marvel Girl" it was not uncommon to see her grow faint from too much mental exertion or input. But with Emma's reframing, perhaps that's an indicator of how much psionic capacity Grey actually has. The Marvel Universe is full of incredible psychics, but as time goes on it seems clear that no one can quite match the X-Men's Jean Grey.