I'm trying and willing, Austin, but I think it's very difficult. Some were pioneers like the Wright Brothers, but they paved the way and this has to be respected. In the UK Darwin comes up a lot, due to his great achievement in science. Albert Einstein too for the same reason as well as his genius.
Both Gorbachev and Mandela for their selflessness and sacrifices; Mohamed Ali for his contribution to blacks, yes, but to the whole world in sport, athletics and simply being a good human.
Rosa Parks and Martin Luther King Junior, perhaps paved the way for Obama, and at one point in his first term, he was the most influential black American in Europe, nay, in America too. I don't think that his achievements should be scoffed at.
Thomas Jefferson is probably my man in politics and Ali in sports. Einstein in Science. Jefferson and Emerson had tremendous influence on so many Americans, and again I return to this idea of Pioneers as well as Patriots.
Women? Sojourner Truth; Mary Seacole the great nurse and humanitarian only just coming to the fore, and finally Emily Dickenson for her poetry and influence on international and national thought. There are many more, obviously.
Let me just say that goodness and greatest can be measurable in the sense that all those we hold good are usually considered great, but not all great considered good. Food for thought? A decent question but not so straight-forward to judge for me. Much Love.