With so many mutants sharing similar abilities in the X-Men franchise, it's no surprise that telepathy is among the most common powers. From Professor X to Jean Grey, there is absolutely no shortage of telepaths in the Marvel Universe to stop such threats as Apocalypse and Mister Sinister.
Whether they inherited their ability from a previous generation or have learned to master their telepathic powers through pure discipline and experience, there are some psionics that are so skilled that they managed to reach Omega-level status. Taking this into consideration, the burning question of who are the strongest telepaths then becomes a topic of interest and debate among comic book fans.
Cable
Nathan Summers is the time-traveling son of Madelyn Pryor (Jean Grey's clone) and Scott Summers (Cyclops). He was born a healthy baby until he was kidnapped by Apocalypse and infected with the techno-organic virus. Like his mother's donor, Cable's mutant abilities include telekinesis and telepathy.
A gifted telepath, Cable's greatest achievement as a psionic is the constant use of his powers to control and keep the techno-organic virus from killing him. Such a feat certainly takes masterful concentration and considerable power to prevent the virus from consuming his body. Truthfully, Cable's limited strength stems from the TO virus. He should actually be far more powerful than he appears if it weren't for the necessity to devote a portion of his power to battle the virus.
Legion
Legion is the multi-powered mutant son of Charles Xavier and Gabrielle Haller. After a terrorist attack greatly traumatized him, David simultaneously developed both his mutant abilities and dissociative identity disorder. Among David's numerous abilities is telepathy, which is controlled by a personality named Jemail Karami -- the name of the original terrorist leader. As Jemail, he has the ability to absorb minds and even destroy them, as he did to the terrorists. Though Legion is an Omega-level mutant, he doesn't always have the best control over his powers, which is why he's not as effective a telepath as others.
Psylocke
The British adventurer originally named Betsy Braddock -- also known as Kwannon, after her mind was transferred into the Japanese assassin's body, is one of the best-known telepaths in X-Men. What makes her strong is her ability to create psionic weapons, which can affect other telepaths' minds when struck by one of her psychic blades. Though she was originally an Alpha-level mutant, she managed to move up the ranks once Jean Grey entered her mind in Uncanny X-Force #17 -- allowing her to reach an Omega level.
Exodus
Born originally in the 12th century, Bennet du Paris first discovered his mutant powers as an adult after Apocalypse had turned him into the villain Exodus fans are familiar with today. Exodus has various powers, including flight, healing, telekinesis, and telepathy. He is a powerhouse when it comes to his telepathic abilities as he has been able to mentally overpower Jean Grey, Scarlet Witch, and the Inhuman Crystal simultaneously.
Even Psylocke admitted he's a stronger telepath compared to her. Not only that but he was able to telekinetically and telepathically rebuild Professor X's brain from a gunshot wound. However, he did once lose a telepathic battle against Xavier, which suggests he's not as powerful as other mutant telepaths.
Kid Omega (Quentin Quire)
A former student of Xavier's School for Gifted Youngsters, Quintin Quire is highly intelligent and among the strongest telepaths. Feeling ostracized during his early tenure at the school, Quintin consumed one of Marvel's most powerful performance-enhancing substances called Kick, multiplying his powers by five in order to rebel against Xavier.
In regards to his telepathic abilities, he is capable of creating psychic constructs, similar to DC's Green Lantern. Though it was originally unclear if his formidable powers were the result of his usage of Kick, his own Omega-level abilities were proven later, however, as he would eventually become the host for the Phoenix Force.
Emma Frost
A former member of one of the most evil X-Men villains the Hellfire Club and known also as the White Queen, Emma Frost is one of the most formidable telepaths in the X-Men franchise. Alongside her telepathy, she has the ability to turn her body into a diamond-like form, making her unable to take damage and preventing other psionics from reading her mind.
Though a strong form, Emma's diamond exterior renders her unable to use her own telepathy, a significant weakness that prevents her from taking full advantage of her first and secondary mutations. Generally, she's categorized as an Alpha-level mutant, but telepathically she is an Omega-level that is practically on par with Professor X. She's not as strong as others as she has frequently been unable to defeat Jean Grey in the Astral Plane.
Professor Xavier
Perhaps the most famous telepath in all of the X-Men franchise, Charles Xavier is an Alpha-level mutant with Omega-level telepathy. He's the founder of Xavier's School for Gifted Youngsters and the leader of the X-Men and the Marvel Universe's foremost expert on psionic abilities in mutants.
Perhaps the most disciplined telepath, he has numerous telepathic abilities including, freezing time around him and being able to reach every mind on the planet via Cerebro. He's usually considered the strongest telepath due to his experience and masterful control of his powers, yet in power alone, he falls a bit short by comparison.
X-Man (Nate Grey)
In the alternate timeline of Earth-295, i.e. the Age of Apocalypse reality, Nate Grey (X-Man) was born via a test tube. He was created by Mister Sinister using genetic material from Jean Grey and Scott Summers. Much like his "mother," he developed telekinesis and telepathy.
Though as Sinister expected, the mixture of Jean and Scott's genes created a child with god-like powers. His telepathic abilities include being able to communicate with everyone on earth and even to a degree into space with little effort. He's also able to read multiple minds simultaneously and communicate across the multiverse as displayed in X-Man #74.
Prestige (Rachel Summers)
Prestige or Rachel Summers is Nate Grey and Cable's alternate timeline sister, and yet another gifted, god-like telekinetic, empath, and telepath. Rachel was born in Earth-811 in the Days of Future Past reality, which contains one of the best alternate versions of the X-Men.
Much like her brother, she has vast and unique telepathic abilities, such as being able to alter a person's pain and pleasure centers of their brains and transfer her mental awareness to other variants of herself in the past and future. Unique to her familial psionic power set is the ability to manipulate time, which she can do for short periods albeit under extreme duress. Like her mother before her, Rachel has also been a host to the Phoenix Force, which enhances her already godly powers tenfold.
Jean Grey
It is no surprise that the strongest telepath in the entirety of the X-Men franchise would be the mother of three of the strongest psychics. Jean Grey has practically no limits to her powers, particularly when hosting the Phoenix Force. Jean is an Omega-level telepath and like others, has a vast array of powers including, mind transferal, memory erasure, increasing and decreasing other mutant's powers, personality alteration, and can perform psychic surgery. Though it's taken a long time to master, she is the only Omega-level telepath to have ever been able to control the Phoenix Force.