Uncanny X-Men - Chris Claremont VII.

X-Men v1 #129-134.

Chris Claremont's run on (Uncanny) X-Men is regarded by many as the best the X-Men have ever been, and accordingly, the Dark Phoenix Saga is regarded as the absolute apex of his work. Mutant X and Days of Future Past are two other storylines cited as standouts. There is some talk of decline in his later years which I don't think is unfounded. At the end of the day, he's only as good as his collaborators. John Byrne is freely credited as co-plotter, and the exact extent of their relative contributions isn't exactly clear. At least in the case of this story though, you should mentally substitute “Claremont and Byrne” in most cases where I type “Claremont.”

The Dark Phoenix Saga's ending is the one X-Men scene everyone remembers, and the whole thing has more iconic moments than you can shake a stick at. Being such a focal point, it's also been hit hard by certain retcons, but not to the extent that it's been ruined. And keep this in mind: as I've tried to emphasize, Claremont excels in long-format stories, with plenty of foreshadowing of the future and referencing of the past. As such, even a compact ten-issue arc like this is reliant on the greater context surrounding it. It's 'only' the payoff of something Claremont and Cockrum started over three years earlier.