While it's quite possible that among all news sources, Jonathan Cook seems to have worked for the one horrific news source in the world, and is doing us a service by constantly exposing their acts, Occam's razor would suggest otherwise.
I've read a sampling of his articles, and he seems to be either an extremist, or someone who wishes to profit by appearing to be one. His posts seem mostly to be lacking a bit in rational discourse (with gems like "As far I can tell, none of the facts in the Guardian’s story is untrue. But that does not stop it from being a blatant lie"), and without much diversity in subject.
That is rather unfair. He's not likely to write about his experiences at e.g. the Mail, because (so far as I know) he never worked there.
I have read his articles for many years now. Most of them (mainly on Palestine) are interesting and (so far as I can tell) accurate. I would say that he is a respectable journalist, and probably a darn sight better than most of those in the mainstream media today. Doesn't mean I always agree with him, or his (usually rather left-wing) political views.
There was a time when journalists like Cook could make a living writing for a mainstream newspaper, but these days there is no place for his reporting on e.g. Israel in such places as the Guardian.