4. Jean Grey/Marvel Girl/Mind-Witch/Phoenix - 587 points (14 first place votes)
Charles Xavier met Jean Grey when she was a young teen, as her powers had manifested themselves when her friend was hit by a car - Jean linked minds with her dying friend, and her friend's death almost killed Jean - sending her into a coma. Xavier put blocks upon her powers and eventually got her back to the waking world. Ultimately, as Marvel Girl, Jean would become one of the original five X-Men (and if only it weren't for that blasted wheelchair, Xavier would be all over that young, nubile girl).
Jean stayed with the team for quite some time, until the All-New, All-Different X-Men showed up. She took a break from the X-Men, but when all the X-Men were kidnapped by Stephen Lang, who is ready to unleash a whole pile of Sentinels from space, Jean found herself along for the ride.
As the X-Men were escaping from Lang's space station, they found that their space shuttle was damaged. Someone had to pilot the ship, but that someone would almost certainly be killed by radiation. Jean decided to do it, and when the shuttle landed, Jean emerged as a NEW being called the Phoenix!
The Phoenix was pretty damn powerful, but after being messed with by the Hellfire Club, the Phoenix's powers got out of hand, and went on a murderous rampage in outer space.
The Shi'ar Empire tried to stop it, but ultimately, Jean Grey decided to sacrifice herself rather than become the evil Dark Phoenix again.
However, years later, it turned out that, ever since that space shuttle crash, the REAL Jean Grey had been healing in an energy cocoon where the shuttle crashed! The Phoenix only was a COPY of Jean, which makes the fact that it was STILL willing to sacrifice itself pretty damn impressive.
Jean became a founding member of X-Factor, where she was too distracted by all the changes since she was away ("What is this fascinating show about the two cousins? Why, one of them is naive and kind and the other one keeps trying out hilarious schemes - oho - the scheme failed!! Haha!") to notice that her former flame was married with a kid - that caused some issues.
However, she stayed with X-Factor until the team merged with the X-Men to form a BIG X-Men squad. Initially assigned to the Gold (i.e. the "loser squad"), Jean soon took a bigger role at the school, especially when she married her teammate Cyclops and the two became co-heads of the X-Men's school for mutants. Their marriage was almost as filled with passion as Bill and Hillary's. I'm surprised they didn't sleep in separate beds like Lucy and Desi.
Awhile later, we learned that Jean is still connected to the Phoenix Force, through whatever means, I dunno, to be honest.
Sadly, Jean discovered her husband was cheating on her with Emma Frost, and soon after, a defeated Magneto added injury to insult as he lashed out one last time - killing Jean Grey.
The Phoenix Force has shown up a few times since then, so never say never for Jean coming back!
Although she is never coming back.