The best writer of a long running series Hellblazer was Jamie Delano. There were a list of great writers upon the series, and to name a single one as best would seem foolhardy, especially since I find many of the people to be favorites in other areas, or at least all around great writers. But Jamie is better as a writer of this kind of darkness, and he does so without any sort of apology. He is unashamed, brash, and able to speak the language of vulgar without accent. I have pointed to his work elsewhere, but do so again with this work specifically because it shows the truth of my comments.
Warren Ellis, Grant Morrison, Garth Ennis, Paul Jenkins, Brian Azzarello, and Mike Carey all kick my ass with awesome, but Jamie runs circles around the bunch. I like the entire series, mind you, but in a way since Jamie was the first, it was a bit of catch up after he left. The reason I am doing this now, regarding Hellblazer is that there have been two attempts to make Hellblazer/John Constantine appeal for Film and TV. It is important to read the best works of the series and realize, John Constantine isn't pretty, he doesn't have proper teeth, he isn't gentle, he isn't well groomed. He is a survivor, he has been a warrior who walks the razor's edge of sanity. And neither the Keanu Reeves of film nor Matt Ryan of television work. Both are too pretty.
Anyhow, read any of the Hellblazer runs, and as with most literary vs. movie comparisons, the depth of the written will destroy the visual and simple of the film.