Jean Grey (Introduced: 1963)
Jean Grey holds the distinction of being the first female X-Man (at least when we ignore all retcons and Sage). Though very “generic female” in the original X-Men comic, she came into her own during the 1970’s. Her evolution into the infamous Phoenix as depicted in “The Dark Phoenix Saga” became the most influential X-Man comic of all time, and arguably one of the most important in comics in general.
Storm (Introduced: 1975)
Being part of the X-Men rebirth in the All-New, All-Different relaunch in the 1970’s, Storm is partly responsible for saving one of the most popular franchises today. Storm was the female X-Men of color, as well as their first Black member. Being first seems to be Storms lot in life, as she was also the first female X-Men to lead the team, as well as one of the earliest women to act as leader for a superhero team. She became the first of many strong female X-Men introduced in the 70’s and 80’s. Plus, she rocked an awesome mohawk in the 80’s, which has thankfully made a reappearance in modern times.
Mystique (Introduced: 1978)
Like Jean Grey and Storm, Mystique was part of a milestone moment in comics. She played the central antagonist in the original “Days of Future Past” storyline, which was become the second most reference X-Men story after “The Dark Phoenix Saga.” She was also one of the earliest bisexual characters, even if her relationship with her lover Destiny could only he hinted at in the early 80’s. A master at stealth with years of experience, Mystique has often proven more than a match for multiple X-Men teams.
Kitty Pryde (Introduced: 1980)
While not personally a fan of Kitty Pryde (Jubilee fo’ life), I can’t deny her impact on the X-Men. Kitty Pryde was part of what you could call third rebirth of the X-Men. A young mutant joining the X-Men after their encounter with the Phoenix and lose of Jean Grey and Cyclops, Kitty helped bring new life to the team and the franchise. Constantly being tested with increasingly dangerous situations, Kitty gave us no choice but to root for her. She served as a bridge for a whole new generation of readers, even if her crush on Colossus was kinda gross.
Emma Frost (Introduced: 1980)
Although Emma Frost was introduced in the 1980s, she wouldn’t become a key X-Men figure into the early 2000’s. A super villain infamous for her role in the downfall of Jean Grey and constant harassment of Kitty Pryde, her inclusion in the X-Men gave the team a delightfully evil member. Always ready with a witty remark and what I can only image several carefully positioned tape pieces, this surprisingly effective teacher helped bring the X-Men into the new millennium.