Robert Capa - 1936 - Falling Militiaman, Spanish Civil War. "Loyalist Militiaman at the Moment of Death". Despite being one of the photographs that changed the world, the validity of the photography has been questioned. "Shadows of Photography"; a book written in dedication to proving this image staged.
The Power of One -2007 - Oded Bality Jewish woman deying Israeli Security forces as they remove illegal settlers in the West Bank. I don't think she's actually holding anyone off, she ran into the shield and the photo was captured at the perfect time.
1994 - Kevin Carter - Sudan Famine UN food Camp. Perhaps the most famous photography of the century. "The vulture is waiting for the child to die so that it can eat him. This picture shocked the whole world. No one knows what happened to the child, including the photographer Kevin Carter who left the place as soon as the photograph was taken. Three months later he committed suicide due to depression."