Wow, a lot of Great choices here folks. And, I’d go with “Dark Knight Returns” or “Killing Joke,” but those books are already well represented, thanks @MatthewHecht and @TheSandmanofGotham!
I want to say “Crisis on Infinite Earths,” because I’d never read a series quite like that before. Ok, “Marvel’s Super Heroes Secret Wars,” was amazing, but not nearly as Universe shattering, those series showed me what comics could be on an epic scale as a child.
But, if you want to get personal, I guess, that I’d have to go with, , “Watchmen.” Ya, gotta admit it’s kinda boring, and typical, but that series blew my mind! I was too young to read it when it came out, but my brother is eight years older than me, and we made a deal. I’d pay for the comics out of my allowance. He’d actually buy them, cause Mom didn’t want me having them. He’d get em, read them first, then pass them on to me for my collection.
He didn’t like them, because he didn’t understand them, neither did I, but I knew they were something special, something different. I read and reread those issues. Then, when I was a little older, I got the Collected Edition, and read that over and over, trying to make sense of it. I have to say, that book changed my life.
Not only altering my view of “Superheros,” but also, a nice introduction to the harsh realities of life in general. Kind of like “The Dark Night Returns” and “The Killing Joke,” and as I got older, I realized how much these series introduced me to story telling and character development. The '80s were a darker time in the industry for sure, but boy, we got some great comics!