You can't find a more influential comic book than Watchmen.
Watchmenwas a 12 issue comic book limited series released from 1986-1987 that received critical acclaim upon its release. It was the first comic book to be seen by mainstream audiences as a legitimate story which dealt with several serious topics and themes. It wasn't just a silly picture book that you would buy for your kids at the grocery store for a quarter, this was a comic book for adults. Of course, there were other stories that came before Watchmenthat also dealt with serious topics like alcoholism, drug abuse, and the clash of political ideologies, but Watchmenwas the one that people outside of the comic book community took notice of. To this day, Watchmenranks as one of Time Magazine's 100 greatest novels ever made, standing alongside classics like A Clockwork Orange, The Great Gatsby, Animal Farm,and To Kill A Mockingbird.
For the comic book community, it, alongside The Dark Knight Returns, ushered in an era of comics where characters became darker, more serious, edgier, and full of 90's...ness. Referred to as "The Dark Age," this time period was one of the worst periods ever for the comic book industry, culminating in Marvel's bankruptcy, but none of that was because Watchmenwas a bad story. Quite the contrary. Watchmenwas so popular that people misunderstood why it was as successful as it was. Many prominent comic book creators believed Watchmenwas successful because it was a dark and mature story, so they tried to emulate that style without understanding that the content of Watchmenis what made it so good, not just because it was aimed at adults. Watchmenwas good because it was a good story, one that comic book creators still look to for inspiration to this day. The legacy of Watchmenis undeniable, safely secured in the pantheon of comic book greatness.
And then Zack Snyder made an adaptation of it in 2009.