The first rule I think is most important for journalists to follow is accuracy.
"In the end, the discipline of verification is what separates journalism from other forms of communication (...) journalism alone is focused on getting it right," Bill Kovach and Tom Rosentiel of the Project for Excellence in Journalism, All the News, chapter 14. Photo from, ontapblog.com. Jason Blaire, known for fabricating stories.
In my opinion accuracy is the most important because if you are accurate as a journalist, truthfulness, thoroughness and fairness will be a bi-product of it. The public expects journalists to be accurate. If they are not, I don't see the point in journalism.
The second is 'being forthright about mistakes.' We are only human and error will occur, I believe that bringing that to attention rather than covering it up is a sign of a ethical journalist.
The third is an ethical issue and that is deception in gathering information. To allow your subject to disclose details that they would withhold if they knew who you were seems so dishonest. The story produced by such methods would just feel wrong and fake.