Fannie Lou Hammer because she stood before all the others. She received less notice but is a giant before others like Oprah, etc. Like she said, "I'm sick and tired of being sick and tired."
I chose Madame C.J. Walker because she knew the power of entrepreneurship & how it's important to build wealth within the African American community. She did the unthinkable for her time - an African American AND Woman successful business owner despite the crushing humiliation of racism. Without her tenacity and resolve, there would not have been an Oprah Winfrey, Russell Simmons, Sean Combs, etc.
Collectively they represent the strides Black women have made in this country. Each of their achievements were so different, including Oprah Winfrey in the field of communications, their accomplishments can't be judged against each other.
I vote for Mary McLeod Bethune. http://en.wikipedia.org/wiki/Mary_McLeod_Bethune
Oprah Winfrey - came from a place of poverty and abuse. Yet she never let that hold her back. She used that adversity to make her and those that she touches better. She keeps showcasing new people and new ideas to make things better for all (men, women, and children), and her generosity cross over to all ethnicity.