5) Bishop
After the runaway success of the Days of Future Past storyline, the X-Men comics were repeatedly cluttered with alternate-timeline/dystopian future arcs. Not all of them worked, but one great thing that came from the trend was Bishop.
From another future where Sentinels have enslaved the world, Bishop is a member of Xavier’s Security Enforcers, a kind of mutant police force and successor to the X-Men. He’s accidentally sent back to the past where he joins the original X-Men and works to avoid the terrible future he knows from ever happening.
One of the best examples of the mullet-headed, anti-authoritarian, anti-heroes of the comic books of the 1990s, Bishop’s popularity has lasted into the 21st century. He even recently made an appearance in the movies, albeit in a very limited role, when Omar Sy played him in X-Men: Days of Future Past.