The one every later X-Men story is measured against. Jean Grey’s power keeps escalating, the team keeps rationalising it, and by the time anyone admits what is happening it is far too late.
What makes it work is that the tragedy is structural rather than villainous. Nobody involved makes an obviously wrong call. They each make a defensible one, and the defensible calls add up to a catastrophe.
The ending holds up because it was not reversed at the time. Everything since has spent decades chipping away at that.
Read October 2026.