The Beginning of The End
With the Changing of the Guards, makes a change in the tone of a comic many of times. With Jamie Delano, Delano fleshed out John further into Magick and its relationship with the human mind, while Garth Ennis grounds John to his Human roots and questions life and the meaning of it.
What I enjoyed most about Garth Ennis' Hellblazer is that The Preacher formula is no where from being the level it is now. Garth Ennis was a virtual nobody at the time and I think that "innocent" charm to his take of John and I used the word innocent loosely, was what made his impact on John so pure. Ennis makes sure to address the major shift from Delano's run by talking about the Twin and the events of The Hang Man and Magnus issues #39-40.
Those who watch the Constantine (2005) movie will notice the idea of John getting Cancer and the debate between suicide was highly influenced by the Garth Ennis run. The run starts with The Beginning of the End and you can read this series in Hellblazer: Dangerous Habits or this issue alone in the movie tie-in trade Constantine: The Hellblazer collection.
For any John Constantine fan and/or Garth Ennis fan Dangerous Habits is an important arc to read.