I would say that the comics is the souce text because it is created the earliest by Alan Moore. The movie is a adaptation since it adoptes the original characters and settings but changes form and explores deeper meanings. And as for HBO’s show, I would say that it can be characterized as an adaptation but it also stands on its own. If you are already a fan of the graphic novel, you’ll appreciate the intertextual elements portrayed in the series, and if you’ve never read it, then it still works as a complete and independent science fiction. And I remember seeing some reviews from bbc and the guardian about the show being a great adaptation, and I can see their points despite the fact that the show is quite different from the source text.