Mutant Massacre
Fall of the Mutants
Inferno
X-Tinction Agenda
X-Cutioner's song
Fatal Attractions
Phallanx Covenent
Age of Apocalypse
Onslaught
Operation Zero Tolerance
Messiah Complex
Second Coming
Age of X
Battle of the Atom
Other
I thought X-Tinction Agenda was an excellent read. Generally speaking, I really enjoy sci-fi stories that speak to the world around us, or at the very least, draw influence from contemporary events.
At the time, Claremont was really on his game. In my humble opinion, he used the concept of a fictional island nation (Genosha) to tell an interesting story and highlight the injustice of being considered a second-class citizen in your own country. I know the plot featured your standard superhero fare (i.e. spandex, bright primary colours, etc.), but I still hold it in high regard.
Genosha is clearly intended as a fictional stand-in for Apartheid-era South Africa and the crossover holds a mirror up to what was an inhumane and cruel regime. I thought Claremont did an excellent job of demonstrating the differing points of view on the island i.e. not every human depicted was an unapologetic bigot. Some were very committed to overthrowing the regime and ending discrimination and injustice. Others were utterly wedded to the idea of racial superiority. For me, the complexity of these competing viewpoints and ideologies made Genosha "feel" authentic and real, as opposed to a simplistic "cardboard cut-out".
Also. this is probably cheating (as it's not actually an X-Crossover strictly speaking) but I've always had a soft spot for BloodTies
Messiah Complex and Second Coming work well together in an Infinity War / Endgame way.
Mutant Massacre is my favorite cross over. X Tinction Agenda and Inferno were also fun to read. Kings of Pain and Days of Future Present as well.
HoXPoX is the best event book, Inferno is my favorite classic crossover, and Age of X is my favorite modern crossover.
1. AoA
2. Onslaught
3. Second Coming
4. Fatal Attractions
5. AoX
6. X-Cutioner's Song
7. X-Tinction Agenda
8. Messiah Complex
9. Phalanx Covenant
10. Operation: Zero Tolerance
Yeah I have a hard time seeing someone vote Battle of the Atom.
AoA is top for me.
Though there's a lot of good ones.
AoA - so fun. Magneto's X-men and the cool new designs. Apocalypse reigning supreme. Alpha and Omega are 2 of my favorite books.
X-Cutioner's Song - I think this was my first real crossover as a kid. X-Force fighting the X-Men then teaming up. there was a lot going on.
Phalanx Covenant - I loved this as it led to Gen X. the short lived Emma/Banshee/Jubilee/Sabretooth team was good. then we got Synch added in.
Fatal Attractions - some memorable moments with Magneto pulling the adamantium and Colossus swapping sides for a bit.
OZT - the fun trio of CeCe, Marrow, and Iceman. plus Maggott!
Onslaught - was fun in the build up but feel the actual ending and Onslaught just being some entity didn't deliver
all fond memories of growing up with.
I'll say Necrosha was probably my most disappointed crossover. It had this potential to be so good. all we got was Cypher ..... and they tried to make him badass when it just doesn't work. It could have been so much more.
The only thing more disappointing I can think of is the Weapon X/Neverland resolution. Though it probably doesn't count as a crossover. We got a damn internment camp with tons of mutants there and this wicked Weapon X program and again the resolution was so disappointing. the X-Men did nothing.
Definitely Messiah Complex, that had me glued to my seat each week for the next issue...x of swords is doing pretty decently.
X-Cutioner's Song if only for the Wolverine, Bishop, and Cable team-up.
I'll always have a soft spot for X-Cutioners song since that was the 1st big crossover I read as a kid. Hoping X of Swords turns out good, haven't had a real good crossover since Second Coming.
My vote goes to X-Tinction Agenda, with Inferno a close second.