Uncanny X-men #100 ended a three-part saga culminating in the crashing of their spacecraft and the subsequent death of Jean “Marvel Girl” Grey. She would erupt from the wreckage as Phoenix and spawn decades of continuity in the process.
Big fraggin’ deal. The truly historic moment of this issue was the execution of the now-legendary fastball special! That’s where Colossus grabs Wolverine and throws him at something far away that needs to be destroyed right the hell now.
Collector’s Guide: from Uncanny X-men #100; Marvel, 1976.