system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel/DC] What single event (like doomsdays, extinctions, invasions, etc.) has had the largest impact on the respective universes?
gpt
DC - The Crisis on Infinite Earths.  Literally to the point that events in DC are always referred to as "pre-Crisis" or "post-Crisis".  It involved the Anti-Monitor destroying countless worlds, with many major heroic deaths and long lasting impacts.

Marvel - Probably the Incursions that led to the Secret Wars.  The entire multiverse was destroyed and only managed to survive at all because of Dr. Doom and Dr. Strange.  There were similarly large scale ramifications, but this is less clear cut as THE biggest thing than DC's is with Crisis.