When I think of the X-Men this is the image that first comes to mind. The best line-up. The best creative team at their at-each-other's-throats creative peak. This is the defining story along with, for my money, "God Loves, Man Kills" close on it's heels. Twenty-five years later, and it's still a key foundation of the X-men mythos.
When I first read this story it was in this graphic novel format with this cover, and hot on it's heels came Uncanny X-men #175 a more than fitting follow-up tale.