First, I understand that the nitpickers out there will observe that this picture isn't really of Jean, but of Phoenix at the instant of her self-immolation. I've chosen this image intentionally, because that very observation goes to the heart of what I love about the Jean Grey character, the perfection of the Dark Phoenix storyline, and the hamfisted retconning that has spoiled modern comics for me.
Professor X is my favorite of the original X-Men, and we'll examine him later. But Jean Grey, in her Marvel Girl persona, was always the heart of the team. Her relationship with Cyclops was written dynamically and with a remarkably adult perspective, and it gave the strangest mutants of all a humanity in the midst of their strangeness. Originally conceived as the weakest of the team, Jean grew in the telling, and by the time the new team was introduced, she had matured enough to walk away from the super hero biz despite her burgeoning talents as both a telekinetic and a telepath. Then fate intervened in the form of the Phoenix entity, and the most compelling arc in comics history began, bookended by two acts of selfless sacrifice by Jean Grey, the most courageous and loving X-Man of all.
Phoenix joined with Jean while she piloted a doomed shuttle to Earth, knowing she would die while her teammates survived. For the record, I was never on board with the retroactive canon that Jean was cocooned in stasis while the Phoenix replaced her. No no no no. Jean died that day and was reborn, cosmically powerful and yet still human. Her struggle to balance her hunger and her power with that humanity was Jean's struggle, and it is totally cheapened by later changes. The descent into Dark Phoenix was Jean's inability to resist the seduction of power. It's a universal parable about corruption by power. In the end, when Phoenix (Jean) sacrifices herself again by killing herself on the moon, it is the ultimate act of love and courage, and the redemption of her character.
It was brilliantly conceived, written, and was the rare comic book death that was wrenching and durable. It sent shockwaves through the X-Men that would last until Marvel caved and brought her back. I've never accepted the post-X-Factor Jean Grey. To me, she died on the moon at the conclusion of a perfect story.
For the record, while Famke Janssen was a passable Jean, the movies butchered her entire storyline beyond repair. How you screw up the Dark Phoenix saga is beyond me.