If they ever portrayed the character properly, Tim Roth would be my first choice. When I first read Hellblazer, the character I pictured looked and sounded almost exactly like Tim Roth.
https://secure.wikimedia.org/wikipedia/en/wiki/John_Constantine
Only difference is that John Constantine was originally from Yorkshire, while Tim is clearly a Londoner.
Screw Keanu Reeves. He's not even English.
I second this idea. Keanu Reeves was a horrible, horrible choice. If we've got to have an American Constantine-- which we don't, but anyway-- it could only be Denis Leary.
shareNever thought of him as Constantine, but now that you mention it he would be perfect. He's British, as the right age and can play both creepy and charming. Definitely a good choice.
I'm a fan of the comic myself, and I often picture Billy Idol as Constantine. Could be interesting, but I have no clue if the man can act.