The top telepathic mutants go as far as Professor X, Jean Grey, Psylocke, the Shadow King and Emma Frost. The Shadow King is easily the weakest as both Emma Frost and Professor X have beaten him individually, but he was 'in ancient times' considered the strongest telepath in existence and could exist solely on the telepathic plane. This sets the bar. Professor X has beaten Psylocke and Jean Grey even absorbed his powers, so he's null; the only reason he was around was because he could use Cerebro nearly as well as Professor X. This essentially leaves the top spots for Emma Frost, Jean Grey and Professor X.
Now, let's look at these three. Of the three, all of them have existed independently on the telepathic plane, which is an accomplishment in and of itself. Canonically, Emma Frost has beat Jean Grey (in earlier conflicts) telepathically and Professor X was able to lock away the Pheonix Force with several mental barriers to help her control her power. Ultimately, Jean Grey is the only tekeinetic in the group. Professor X and Emma Frost are solely Telepaths. Emma Frost has accomplishments such as taking over and using the bodies of other Omega-level mutants and even using their powers better than them; cite her interactions with Iceman. Professor X is the foremost user of Cerebro, has battled multiple telepathic enemies simultaneously and has resisted the telepathic assaults of both Jean Grey and Emma Frost.
In terms of sheer power, Professor X has canonically beaten Emma Frost, but in terms of utility she can do everything he can - including use Cerebro on a global scale. Professor X is for all intents and purposes the stronger of the two, even if he cannot perform all the same things. This would leave Jean Grey and Professor X as the top spots. Now, here are some facts. Professor X put mental blocks on Jean Grey that neither SHE ALONE -OR- Emma Frost could unlock. Through time and effort, it took five telepaths of at least-alpha level whom also served as the host for the unlocked Pheonix Force AND Jean Grey being manipulated to unlock the barriers on her own mind. Canonically, she had never beaten or excelled Professor X in a telepathic battle; in fact, she lost several.
There is no instance in mainstream or even common source canon that shows Jean Grey beating Professor X BEFORE she unlocked the Pheonix Force and it is entirely unfair to judge what she does afterwards. She has her full potential unlocked, is fueled by infinite cosmic power, has control over life and death and is easily in the top ten most powerful Marvel characters right under entities such as the personification of Death itself. Additionally, during her time as a mutant, she had to train herself telepathically and telekineticaly AND had far less experience than Professor X. In short, even if she had the potential to be more powerful than him, she literally never was. Whether you want to argue she could potentially be more powerful or not, the fact remains that Professor X remained the strongest telepathic mutant in Marvel continuum whether it was for the sake of plot or convenience. Those are facts.
