Take a look at 13 pictures that changed the world. While I know there are others that are equally notable (see the comments there for some ideas), it’s a great trip through the history of photography. I was particularly struck by the split between great art (#7 being a particular favourite of mine) and great photojournalism (not so easy to have a ‘favourite’, but there are few images more powerful than #4, though the pictures of the monk ThÃch Quảng Äức burning himself to death, and the Vietnamese girl Kim Phuc running from a napalm attack would qualify.
I think I might have started the series off with the first photograph ever taken, as that did rather get the ball rolling.
(HT: FreeMacPhoto)