The issues #129-137 contain one of the most iconic stories ever run under the X-men title.
The first few introduce a slew of characters that later become rather big names in the entire X-men mythos: Kitty Pryde, Dazzler, Sebastian Shaw, and Emma Frost. In it, the X-men have just gotten through with one tough mission and are on to another: recruiting two mutants Cerebro has just found. In the meanwhile, Mastermind is working a powerful illusion on Jean Grey, and he and his Hellfire Club go to intercept the X-men. This eventually releases a darker side of Jean and that leads to the rise of a darker side of the Phoenix within her, with a catastrophe on the galactic scale.
The story really does have consequences further on in the X-Men story line, as 30+ years later they still reference it often. All-New X-Men, for instance, has a time-traveling Jean Grey deal with the consequences of this story specifically, and before that she had to deal with Mastermind's own daughter who tried to pull the same trick on her. They even reused the villains introduced like Emma Frost, here somewhat of a minor villain, and transformed her into one of the most important X-Men right now as a leader of the current Uncanny X-Men.
The art here is in the classic 80s style, and still holds up today. The writing is also good, where it can be somewhat of the cheezy comic book dialogue but it can get emotional at times and it does portray the characters well. Overall, it's a great story that has probably influenced every single medium that the X-Men have touched. And with good reason, of course.
Collects Uncanny X-Men (1963) #129-137. Gathered together by Professor Charles Xavier to protect a world that fears and hates them, the X-Men had fought many battles, been on adventures that spanned galaxies, grappled enemies of limitless might, but none of this could prepare them for the most shocking struggle they would ever face. One of their own members, Jean Grey, has gained power beyond all comprehension, and that power has corrupted her absolutely! Now they must decide if the life of the woman they cherish is worth the existence of the entire universe!