Annie Leibowitz just sort of goes with Richard Avedon as she studied with him. Evolution or revolution as A.L. went to an extreme with her portraits. She's capable of very subtle 'straight' documentary work too.

When looking at the Magnum Photogs be sure to remember the fearsome five who started this whole realm of photography as the original partners:

David Seymour, the 'Godfather' nicknamed by all 'chim'
Robert Capa, the inventor if you will of modern war photography
Henri Cartier-Bresson the great street/candid photographer shooting for 'the moment'.
George Rodger, a war photographer know for documenting the London Blitz who would never shoot war again after a time and went to Africa following a childhood dream and sent back images unlike any before.
William 'Bill' Vandivert, the tall imposing great LIFE photographer who would be a founding member of MAGNUM.

Look at
Joel Meyerowitz a visionary of light.
Andre' Kertesz who raised candid and snapshots to a form of its own.
W. Eugene Smith.
Man Ray.

Don't shy away from the pretty girl photographer's of S.I., Playboy, Maxim, Cosmo, etc. This type of work takes incredible skill and lots of work to make it look that good every time.
Helmut Newton is one of my favorite 'classic' fashion photographers.

There's
Harry Benson (photographed the Beatles)
Berniece Abbott
Dorothea Lange (a FSA-WPA photographer)
Jaques Lowe (known for Kennedy photographs)

O.K. modern/current photographers of serious worth:
Mary McCartney Donald
Sam Taylor Wood
Huggy Ragnarson
Nancy Crampton