The best works of fiction are generally untranslatable into different media. It is impossible to make a film that encompasses all the great things about Catch-22. Citizen Kane and Vertigo would make lousy novels. Watchmen is comparable. Many people I know refuse to read it, just because it's a comic. But it couldn't work any other way. I'm trying not to spoil too much of the plot, but Alan Moore's writing, the pacing of the story - he somehow manages to overlay multiple plot lines on just one page. The dialogue is realistic: people talk over each other, conversations are drowned out by TV, the characters stutter and repeat themselves. The characters are real, flawed. the plot manages to mix high-concept science fiction, hard-boiled crime, philosophy, conspiracies, action, drama. Alan Moore isn't just one of the greatest comic writers ever, he's one of the greatest writers ever. His work stands comparison with anyone - Harper Lee, Joseph Conrad, Heller, Orwell.
But Watchmen isn't just about the writing: Dave Gibbons' art isn't flashy, but no other artist could've drawn the story. Gibbons' work is realistic, insanely detailed. Again, I don't want to spoil the plot for you, but analyse every panel. Every detail, no matter how minor, has purpose.
This book is flawless. It will never be surpassed.