Mark Felt.
No one likes a snitch, but Mark Felt, the former FBI agent who late in life revealed himself as the great mystery man known as Deep Throat, performed an act of high patriotism by helping Washington Post reporters Bob Woodward and Carl Bernstein expose the most serious set of political crimes in American history. His identity also became one of the great journalistic obsessions of the 20th century. The obsession with the identity of Deep Throat sometimes took journalists down strange detours even though tracking down a competitor's source is not the highest journalistic calling. At TIME, a couple of us pursued a thesis that we thought led to Alexander Haig. But that proved to be a dead end when Felt started to go public about his role as Deep Throat in 2005. Woodward subsequently published a short book detailing the relationship, providing the most interesting footnotes to a grim historical moment in the country's history.
—John Stacks