Who might be the greatest living peacemaker? I acknowledge the question is a bit impertinent. It conjures competition, while by its nature, the word "peacemaker" bespeaks humility, equality, warm humanity. Even so, for the title of greatest I place my money on Hildegard Goss-Mayr of Vienna. If you don't know of Hildegard, I urge you to get the first biography ever written of her, Marked for Life: The Story of Hildegard Goss-Mayr, written by Richard Deats, published by New City Press.
She was born in Austria in 1930, and grew up in an unusual Catholic family dedicated to peace, even while under the Nazi regime. Early on, she studied the philosophy and practice of nonviolence, and with husband, Jean Goss, became an apostle of nonviolence.