actually if you are referring to an x men member only, with just his or her super power without any outside help such as the phoenix force or cosmic power,,, then the most powerful is nate grey... why? then read...
Nate is an Omega-level mutant[7] who has the ability to tap into the enormous psychic resources of the astral plane in order to manipulate matter and energy at vast scales. This grants him incredible psychic powers including telepathy, precognition, and telekinesis. He can use his telepathy to read and control multiple minds at once and even read residual thought imprints left on objects touched by people (psychometry), communicate with others by broadcasting his thoughts, create illusions by altering the perceptions of others, fire psionic blasts that can scramble an opponent's thought processes (causing the victim either intense pain, or rendering them unconscious), project his mind into the astral plane and even pull the astral projections of other telepaths into the physical world, and sense dimensional rifts or anomalies.
His telekinesis is so powerful he can move massive objects with his mind, fire blasts of psychokinetic energy that can shatter steel, create mental barriers that can stop most attacks, and levitate himself to fly at supersonic speeds. His control over his telekinesis is so acute he can create holograms by mentally manipulating water molecules and dust to refract light, bend security lasers around himself to avoid detection, and even move the atoms of a wall around his own so that he passes through the wall like a ghost. He can even use his telekinesis to bend the Earth's magnetic field and create electromagnetic pulses. His telekinesis extends to at least a molecular level, and he can imbue himself his super human physical attributes by focusing his telekinesis inwards. He also has the ability to teleport vast distances, and has visited alternate realities by breaking the barriers between universes.
An alternate version of Jean Grey, "Queen Jean" described Nate Grey as the ultimate telekinetic with the statement: "It is what all Nate Greys have been on every earth".[8]
Upon examination by Moira Mactaggert, she suggested his psionic powers rivalled that of a Phoenix Force-imbued Jean Grey.[9] In another instance, he was measured as having a psionic energy output matching that of the Dark Phoenix[10]
Since his return Nate has displayed further abilities which include: precognition, the power to view and transverse higher planes of existence, to reconstitute his body from astral energy in a similar manner as Onslaught and to transform his physical body back into astral energy.
It was said there that his psionic energy is on par with the phoenix/ DP... which is one of the most powerful being in the marvel universe...
