I've had the privilege of working with Cig Harvey, Joyce Tenneson and Cheryl Jacobs.
I'll second Fay Godwin. I had the privilege of taking a workshop at Duckspool in 1992 that she was teaching at. Great experience. Sadly, she is no longer with us.
Also Mari Mahr is another photographer that I admire.
Hilla Becher (as one half of the famous team)
Anne Larsen--I have two of her photographs on my walls at home. They inspire me every day.
Laura Gilpin - was fortunate to see the working copy of her last book, which has not been published as of today
Carlotta Corpron - two ladies who's work is quite under rated and a real treat to view..
Laura Wilson - worked with Avendon on the Amercian West project
check them out, they will surprise you I think.
I'll second Debbie Fleming Caffrey. She has taught me so much and I value her friendship. Another modern day photographer that I admire is Joyce Tenneson. I also love Margaret Bourke-White and Dorthea Lange. And many of my unknown friends.