5. Dark Phoenix
Ignoring the terrible retcon, Dark Phoenix is a hero turned villain by having too much power. The X-Men were about to die in a space shuttle that was breaking up on re-entry into earths atmosphere. Jean Grey sent a telepathic message into the cosmos for help. An ancient entity known as the Phoenix Force responded to Jeans call. The shuttle landed safely, and Jean was reborn as Phoenix with greatly increased powers. Due to the corrupting influence of the villain Mastermind, Phoenix was encouraged to use her powers more and more based on her own emotions and desires. Eventually she became Dark Phoenix. Dark Phoenix attacked and easily defeated the other X-Men before teleporting herself through a wormhole to another galaxy. There she consumes a sun for the sensation of experiencing the power, resulting in the deaths of billions of living beings. Dark Phoenix has the telepathic and telekinetic powers of Jean increased to a practically infinite level. She can even use her telekinesis at a sub-atomic level e.g. when she changed a tree into pure gold. The only way that Dark Phoenix was defeated was by Jean taking control of her body and sacrificing herself. Dark Phoenix did appear in X-Men: The Last Stand, but she was a far cry from the near omnipotent being of emotions and desires. For the most part of the film she moped around with Magneto. She did not consume a star, and Jean did not sacrifice herself to kill Dark Phoenix. Instead Dark Phoenix was killed by Wolverine, who she could have easily disintegrated like she seemingly did with Cyclops, her true love.
Likelihood of appearing again There have been rumours that the Phoenix will appear in a cameo in The Wolverine to be released in July. The Wolverine is set after the events of X-Men: The Last Stand and it is unclear whether a Phoenix cameo will be a flashback, a vision in Wolverines mind or a physically resurrected Phoenix. Either way, short of a complete reboot, it is unlikely that we will see a proper version of Dark Phoenix in film.