Tank man [updated]
Source: AP/Je1ff Widener
The image of a defiant man stopping a column of Chinese Red Army tanks heading towards the 1989 Tiananmen Square massacre has become iconic. Two recently unearthed photos give greater context—the uncropped photo is frankly terrifying.
See also "40 of The Most Powerful Photographs Ever Taken"—and scroll down to comments for other contributions and suggestions.
UPDATE: Video here of the massacre and the confrontation, when for one moment the honour of one billion people rested on the shoulders of one man. [Hat tip Riko S.]