I would say most of it is pretty good, though everyone has their preferences.
I don't think anyone who liked the book would object to the first two runs: Delano and Ennis. Delano's is a bit dated with the politics, but he sets everything up. Ennis makes the book and the character his from the first storyline which is probably the classic Hellblazer story with the classic Constantine moment. I actually prefer the stuff later on when Dillon comes on board because he lets Ennis open up tonally beyond the horror stuff. Kit shows up here and I don't think Ennis has really topped her as far as his female characters are concerned.
I also like Azzarello's quite a bit (though it's definitely the most controversial but I will say that it's one of the few Azz comics I like) because he embraces the fact that no one really ever got the voice of John Constantine as pitch perfectly as Ennis did by making John a kind of background trickster figure for his run. Azz is the only American other than the two Jason Aaron issues who wrote the book iirc and it's an odd run partly because of that, but it mostly works.
And (being me) I also liked the Milligan run that ended the book. The guys on the Hellblazer forums hate it but w/e, fuck 'em imo. It's Milligan writing himself basically, an old man who's scared he's past his prime and kind of a bullshitter. Makes a quite funny little scene when he confronts Shade, who's a kind of surrogate for a younger Milligan.
Other than that you got Ellis (I don't like Ellis but if you do this short run might be for you) and Mike Carey (ditto). Never got around to reading Diggle or Jenkins (or Mina, but everyone seems to hate that one)
