The Telegraph runs two lists of the 10 most influential photographs.
10 Pictures that Changed Photography gathers the images that transformed the art like this:

The rest of the list includes images by Atget, Sander, Arbus, Weston, Tilmans, Beaton, Sherman, Frank, Prince, Muybridge and Baldessari.
10 Photographs that Changed the World looks at the photojournalism images that had an impact on the way we undstood events or became events in and of themselves.

Photographs that Changed Photography (Telegraph)
Ten photographs that changed the world (Telegraph)