In preparing for my Introduction to Christian Theology class, I've been reading Alister McGrath's Christian Theology: An Introduction (Wiley-Blackwell, 2011). It's no surprise that St. Augustine towers throughout the book, as the one who developed the early church's thought on sacraments, grace and sin (as with the Donatist and Pelagian controversies), creation, evil, free will, the Trinity, church and state, and so on.
The other surprise, though, is how seminal Karl Barth has been, beyond even the neo-orthodox movement. Barth restored interest in the Trinity, the church, believer's baptism, the problem of evil, natural theology, predestination, universalism, the resurrection, science, and the Bible. Last semester I learned how important Barth has been for Christian ethics, as he was the innovator for what is now the ecclesial school of thought – Hauerwas, Yoder, MacIntyre, Wells, Quash, and others. Barth's influence has been larger than the neo-orthodox movement in theology, or as a major turning point in twentieth century theology; his influence has made him a giant in the theological field, in the category of Augustine and Aquinas. Only time will tell if that influence continues, but at this point it shows no sign of abating. So his reputation is true: he is the greatest theologian since Martin Luther (or, if you're Pope Pius XII, Thomas Aquinas).