Eadweard Muybridge, for his motion studies
Harold Edgerton ('Papa Flash')
WeeGee for his NY street scenes and personalities
Dorothea Lange for her dustbowl images
Nick Ut for his Pulitzer Prize winning
photo showing a young Kim Phuc on June 8, 1972. This photo can still bring a lump to my throat and makes my eyes a little watery. If you don't know the history of this image it's well worth looking up.
and, my all-time favorite, W. Eugene Smith. Probably one of the most powerful documentary photographers of the 20th century - see his Minamata series, or his work on Albert Schweitzer. The photo "
Tomoko Uemura in Her Bath" is possibly one of the most powerful images of all time, showing the results of industrial mercury contamination in Japan.