Watchmen might also be the best comic book on a purely technical level. Moore writes snappy, noir-ish dialogue which fits perfectly into a story about corruption and the dark side of human nature. He also uses unconventional storytelling devices like interweaving the plot with images and text from an in-universe (to use a nerdy term) comic book, which serves as an allegory for the events that “actually” happen. You don’t see weird, nonlinear stuff like this in comics much even today, despite the fact that the visual nature of the story makes it much easier to pull off. Speaking of visuals, the art in Watchmen is great. The artist does stuff like depicting two characters in a mirror and then having each speech bubble hover over the other character’s head. 

I didn’t like the ending. By which I mean it made me angry. I guess that’s the point, though. Without spoiling anything, I will just say that it is a really weird ending for a supposed anarchist to write. You get the sense that Alan Moore might be a leftist more out of a deep hatred for conservatives than any coherent positive political theory. It fits the comic book itself perfectly, though.