Well, 'Civil War' in the Marvel Universe is very iconic. It pitted superhero against superhero over the Superhuman Registration Act. Then there's 'The Killing Joke' which is a key Joker story in the Batman series, exploring his possible origin and his twisted relationship with Batman. Also, 'Infinite Crisis' in DC Comics was a major event that brought together many characters and storylines, causing chaos and a restructuring of the DC multiverse.
One of the most iconic is 'The Dark Knight Returns' by Frank Miller. It redefined Batman in a darker, more gritty way, showing an older Bruce Wayne coming out of retirement. Another is 'Watchmen' by Alan Moore. It deconstructed the superhero concept and was filled with complex characters and a multi - layered plot. And 'X - Men: Days of Future Past' is also iconic, dealing with a dystopian future where mutants are hunted and the idea of time travel to change the fate.
Some iconic comic book stories include 'Spider - Man: The Night Gwen Stacy Died'. This story was a turning point for Spider - Man, showing the consequences of his superhero life. 'Kingdom Come' is also an important one. It showed an older generation of superheroes coming out of retirement to deal with a new, more violent generation. And 'Secret Wars' in the Marvel Universe, which had heroes and villains fighting on a strange planet created by the Beyonder.