I haven't gotten around to rereading Shadowland yet but Mutant Massacre and Fall of The Mutants, which along with all the preludes and sidebars really can be seen as one massive crossover arc, is pretty incredible and under-appreciated, even if it is kind of three different stories and some sidebars with some common themes, especially with the new trades. I mean it starts with the X-Men after just having had the shit kicked out of them by Nimrod and the Hellfire Club get hurt even worse by the Marauders to the point of going on the run across the country from them with X-Factor being hurt even worse on the inside and out. It ends with the X-Men sacrificing themselves to save reality while X-Factor scores the biggest PR victory for mutants ever by beating Apocalypse and his Horsemen over Central Park.
Meanwhile, Daredevil, Captain America, Power Pack and Thor have to clean up all their messes and collateral damage.
Annihilation was lightning in a bottle imo. It's too bad that Joe Q was so street level oriented as EIC because (anecdotally) that series brought a lot of people back to comics who had all but abandoned the medium. Pretty much nothing will ever top it.
"Infinity" may or may not be good, but one thing for sure is if it all plays out with him seeking ultimate power etc again, it would mean Thanos ultimately had to go through a character regression in order to make it happen. That's no bueno IMO.