oh man. That is a great question.
Days of Future Past was fairly brief, with two, maybe three issues at most, all under the
Uncanny title. The dynamic change from the "usual" everyday mutant experience, although often filled with peril, mirrored normalcy. Then DFP comes along and flips everything upside down. A dreadful bleak future. Dead mutants, even the heavy hitters are gone. A desperate hypertensive secret mission is devised to reverse all that had transpired. It was an excellent time travel/paradox plot.
Not sure if Marvel ever rebooted
The Brood Saga after I stopped buying comics on a weekly basis. I remember the original story, which solely fell under the
Uncanny title like DFP. I will say this as a movie horror fan. That story reminded me so much of
John Carpenter's The Thing. I mean, HOW can the team survive after being abducted and transformed into hideous Alien looking parasites? And yeah. They LOOKED like Ridley Scott's creatures but the transformation process reminded me moreso of The Thing because the eggs they laid inside a host converted the entire body instead of the parasite popping out of the person's body. The story was sublime and awesome.
The Dark Phoenix Saga still takes my breath away. To me, Jean Grey was a beloved character. She was originally the meek Marvel Girl, who matured over time into a well-rounded mutant superhero of her own before the infamous space shuttle mission. Cosmic being and mutant merged into one being. After that new hybrid character take root over a short time, Chris Claremont and John Bryne allowed TDPS to take Jean and the rest of the team down into a hell hole, with the Hellfire Club being the catalyst of Dark Phoenix's emergence. I could be wrong but I think it was the first time the title itself went cosmic. Jean freaking destroyed an entire solar system. An
ENTIRE solar system. Like it was nothing. Xavier and Beast got her mind back under control just in time for the Shi'ar to swoop in and put The Phoenix on trial for genocide. Jean makes the ultimate sacrifice in the end, saving Scott and the everyone else (maybe in the entire galaxy or more) from utter destruction. I am almost tempted to say TDPS was the biggest except...
Age of Apocalypse.
Age.
Of.
Apocalypse.
Marvel Comics had this idea. Place the X-Men at the center of its universe. Have events warp reality itself into a totally different one even more bleak than what was seen in DFP. And connected
EVERY <expletive> X-Men publication title to that story.
No X-title (What? Seven? Eight? I do not remember exactly how many publications got lumped in) was spared. Every character in the "normal" universe was twisted under Apocalypse's shadow. Good guys became bad guys. Bad guys became good guys. So much to discuss. Do not have that kind of time though lol. Little things were fantastic. Like. How the HECK did Wolverine end up one handed?????? Oh. Cyclops optic beamed that hand into nothingness that's how. And that reveal did not surface until well into the saga that last an ENTIRE year. TWELVE months. 12. And his Horsemen! All bad
<expletive>. And the main man himself, Apocalypse, was so utterly evil, not only wiping out the whole homo sapien race in favor of homo superior all over the North American continent and was about to expand his genocidal campaign regularly across the globe before he was stopped. He was even more cruel towards mutants he considered weak.
And all this happened without Charles Xavier.
Biggest? In my opinion, it's AOA. Final answer.
Edit: Left out Byrne's name again. What is WRONG with me?