X-Men: What Happened To Everyone Who Became The Phoenix?
1. Jean Grey
The 'nobody stays dead in comic books' trope is something that fans joke about whenever anyone comes back from the great beyond, and nowhere is this more true - or more relevant - than in the case of one Jean Grey. She's killed off and brought back to life so many times that it's stopped being funny, and it always has something to do with that pesky Phoenix Force.
She first encounters it when the X-Men are returning to Earth and their shuttle is damaged. Piloting it back safely she's exposed to lethal amounts of radiation, and as she cries out for help the Phoenix Force answers. Now, you should all know what happens from here. She goes mad, becomes the Dark Phoenix and kills herself to protect her friends. But this isn't the last time we shall see Jeanie - oh no.
Being brought back to life a second time, she becomes fully connected to the Phoenix Force while she's battling Xorn. When he's defeated she then ascends to a higher plane and it looks like she's gone forever. But wait, there's more.
She pops back up again during Endsong before she returns to the 'White Hot Room', and seen once more during the 'X-Men Red' run, where she's resurrected by the Phoenix Force before she manages to talk it out of bringing her back to life anymore and it leaves back into space.
In fact, the Phoenix Force is as sick of this as we are, so when the time-displaced version of Jean sits down to have a pow-wow with it and demands it resurrects her, it does just so it can get rid of her (until they inevitably reunite years later).