Passing over the four photographers I've already heard of; Robert Doisneau, Marc Riboud, Bill Brandt and Ansel Adams I've chosen this photo by Philip Jones Griffiths (1936-2008).
Griffiths joined Magnum as an associate in 1966 becoming a full member in 1971.
I chose this photo because it draws me in to ask questions. Why is a woman in a graveyard staring so intently at some black thread? Looking closer you can see the hand of a man in the bottom-right holding onto the same thread. What's the back-story here? There's no detail on what it features other than to say it's a girl in Brompton Cemetery. I want to know more, and that's half the battle with making an interesting photo.
You can read more on Griffiths over at his Magnum Profile Page.