X-Tinction Agenda (Uncanny X-Men #270-#272/New Mutants #95-#97/X-Factor #60-#62) is the model X-Men crossover. Several different teams come together to fight an existential threat towards the X-Men, something bigger than usual. Mutant Massacre had kind of tried this but the interlinking was weak; Fall of the Mutants was basically only a thematic crossover, and Inferno had too much weird stuff going on.
X-Tinction Agenda has a nice simple premise, and then throws 9 issues at exploring it, giving ample time to its regular line-up of characters. It has the "X-" pun, which would go on to become a vitally important part of X-Men storyline nomenclature, even to the point where it didn't make sense ("Avengers: X-Sanction").
So, what's this about? The Genoshan government finally goes a little bit too far, by abducting some of the New Mutants (and Storm) from Westchester and forcing them to become slaves. This allows the various teams of X-Men in existence (Cable and the nascent X-Force, the Gambit/Forge/Banshee X-Men, X-Factor, and the Wolverine/Psylocke/Jubilee group), to openly retaliate, with the backing of the US government, and endorsement from their superhero mates such as Reed Richards.
I was a bit sarcy about this on twitter. It seemed like the X-Men comics were finally taking the brave step of attacking apartheid in 1990, just as Nelson Mandela was released. But then I kept reading, and discovered that the ending is not just that the X-Men are able to recover their friends, but that the Genoshan regime has actually been overthrown. Change can happen. Life is not stuck in the status quo. It's worth pushing, as if you don't you're never going to make a difference.
There's changes to characters, too. Warlock changes to dead, for example. Wolfsbane and Storm were enslaved and brainwashed (I don't know that there's a word in English strong enough for what happened to them), and Havok, who had become a Magistrate on Genosha because of the Siege Perilous, is released, and stays behind to rebuild.
I'm sure it won't last, but for now the X-Tinction Agenda is the X-Men's greatest victory for mutants as a whole. Everything they've done is firefighting of one sort or another. They've defeated evil mutants from carrying out evil plans (Magneto and Mystique stand out), they've defended mutants from humans carrying out evil plans (sentinels), and they've protected the Earth in general from threats. But this, despite the losses, is a victory to be proud of.