Can't resist this one.
Robert Frank, obviously, since he is universally acknowledged as the most influential photographer of the second half of the 20th Century. (alive, but no longer active. He said what he wanted to say in "The Americans" and then pretty much moved on. I admire that.)
Any of the New Topographics photographers, but especially Stephen Shore, Lewis Balz and Robert Adams. Adams and Shore I admire for their writing as well as their photography. Good photographers are seldom good writers. They are both the exceptions.
Elliot Erwitt. One of the few photographers to master visual humor.
Jerry Uelsmann, Emmet Gowin, Nathan Lyons, William Christenberry, Lee Friedlander, William Eggleston, Joel Meyerowitz.
Many others, but that's a good place to start.