We must begin with the recognition that there are no sources actually contain. As long as humans are responsible, it will always be like that. People have opinions and beliefs, and no matter how careful they are, it will always have some influence on what they report and how they report it.
That said, there are several sources to take seriously allegations of reporting the truth and give the public all the relevant facts. These sources admit their own biases and try to police themselves to minimize the impact of partisan.
At the same time, there are sources that are not only recognize their biases, but proclaim them, partly because they thought the biased view of their truth and partly to give viewers and readers of this head, so that they can be a factor in bias when reading or seeing what they present ,
Sources are making efforts to contain including the New York Times, Washington Post, and the three major television networks, NBC, CBS and ABC. I will also include Guardian and BBC in this group. Reuters and AP is mainly neutral in their reporting. They see it as their mission to provide news, not opinions.
The sources expressed their partisan prejudices, including Fox News, MSNBC, Breitbart, Daily Kos, Huffington Post and many other web sites of all stripes. They provide news, but with a strong partisan flavor.
How can you understand all this? What is the best way to learn the truth? Read a lot of resources, with a keen awareness of what they see as their mission. And watch what happens over time. The best sources will help you understand what happened. The worst will continue to be taken off guard and feel the need to rationalize the unexpected.