I've deeply, deeply into comic books and I have to say Watchmen is probably one of the absolute best choices, but I'd actually throw in The Dark Knight Returns as its equal. Those are the two that really made me so absolutely into the medium. Dark Knight Returns sets a standard untouched by any superhero story before or since, truly defining these characters, the actions they take, the magnitude of the consequences and how absolutely realistic it should be, while still maintaining a wonderous fantasy element to bring so much life to it. And then Watchmen obliterated everything related to superheroes with absolute, bone-crushing reality. "Who watches the Watchmen?" is one of the most important sentences written in the last century. So my vote's in for either. I would love to experience either of those again, now nearly a decade later.