The Pentagon Papers were broken by the New York Times. The "Deep Throat" espionage controversy with President Nixon's administration was broken by the Washington Post. American filmmaker Linda Poitras introduced the person responsible for the largest intelligence leak in U.S. history (Snowden) to an American journalist Glenn Greenwald, who previously wrote opinion pieces for various American newspapers, including the Cato Institute. At the time, Greenwald was writing for UK newspaper The Guardian, which at the time and still does maintain a significant office in the U.S. [1]
Further pieces based off of the NSA leaks were written by journalists at the Washington Post. Only after a few months did Der Spiegel write any articles. Oh, and the source of Der Spiegel's leaked documents? American software developer and activist Jacob Appelbaum. Hell, he even wrote their most prominent NSA articles. [2]
American journalism is the most robust source of sunlight for U.S. government secrets.
I get it, one can find all sorts of individual cases of American news outlets letting the public down. But in aggregate, no other country's news organization comes close.
[1] (March 2013) http://www.theguardian.com/info/about-guardian-us/contact
[2] (July 2013) http://www.spiegel.de/international/world/interview-with-whi...