Doctor Gateau said:
IMO, the main thrillery plot Cold War isn't all that. At the time, though, it was pretty special for a super-hero comic to address political issues in such a mature and sophisticated way. This sort of commentary is more common now.
I think it is the most effective super-hero parody ever. It combines a lot of black comedy with very deep probing of the roots of super-heroes, inclduing the commercial forces that lead its domination of the medium (the pirate stuff) and the deeper appeal of the genre. Compare it, eg, Marshall Law (or anything by Pat Mills for that matter) for an idea of how sophisticated its commentary is.
The art, inparticular, is brilliantly deployed. It goes beyond more illustration and uses repeating imagery and shapes to suggest correlations and clever scene changes. You just don't see that level of care and precision in comics.
Greatest comic ever? Maybe not. Greatest *super-hero* comic ever? I'd have to say yes.
I think it would be near-impossible for a film to capture the truly unique elements of The Watchmen, and all we'd be left with is the weak thriller plot. Still, Aronofsky might be the guy to prove me wrong...