Rachel Grey
Okay let's not get into her confusing birth and background, she is basically the daughter of Cyclops and Jean Grey from an alternate timeline (she changed her last name to Grey from Summers after Cyclops betrayed Jean). She is a powerful telekinetic and telepath just like her mother was and is one of the host of the Phoenix Force and seemed to show more control over it than even Jean Grey did (although she also was not strong enough to fully control it). She has taken both the code Phoenix and Marvel Girl at different points in time in honor to her mother (her official code name at this time is Marvel Girl although most just call her Rachel). Rachel is pretty much a boss, she has what seems like unlimited power and ability to grow in her power, she still has a "static" or "echo" of the Phoenix Force within her. Besides that Rachel has shown the ability to pick up new skills extremely fast, perhaps as a side effect of her mind being so advanced with her powers. Another effect that Rachel has shown that is very unique to her is the fact that she has the ability to chronoskim a persons mind to transplant it a younger/older version or relative (most notably in the storyline Days of Future Past), she also has the ability to sense and manipulate psychic energy via psychometry (basically reading an object); she also has a fouth dimensional pulse that she constantly emnates to protect her changes in her current timeline via someone messing with it in some way. All in all she is a very powerful mutant and useful team member, it has been noted that Rachel is an Omega-level superhuman just like her mother albeit that this was from Nimrod (the Sentinel from the future) so this might be something she will become and not yet is.