Karl Rahner was a very powerful and influential theologian, a Jesuit. I haven’t read anything by Hugh Rahner.
There are a number of powerful and compelling Catholic theologians, some of whom are also quite controversial. The most controversial I can think of who was also the most gifted was Hans Kung, who symphasized a lot with the brilliant Lutheran theologian Karl Barth. He also questioned Papal infallibility and teachings on contraception, and he was censured for this and lost his post as a ‘Catholic’ theologian for apparently rejecting some parts of the magesterium. I feel a lot of his criticisms though are spot on and correct, and remain a useful guidepost to future reform in the Church.
Hans urs Von Balthasar in my view is probably the greatest Catholic theologian in the 20th century. He wrote hundreds of books and articles and had an extremely wide knowledge of church history and dogma, patristics, comparative literature, philosophy, and Thomistic philosophy. He also did remarkable and groundbreaking work on recovering the notion of Beauty as a key part in the human experience of the mystery of Being, along with Truth and Goodness. Balthasar’s most daring work is the attempt to write a ‘Theological Aesthetic’ divided into three parts, which include several volumes titled ‘The Glory of the Lord’ which explore Beauty as expressed in philosophy and theology in the Western tradition, Theo-logic, and Theo-Drama.
Balthasar also wrote several key studies of Church Fathers including St Maximus Confessor (Cosmic Liturgy), St Gregory of Nyssa, Origen, St Iranaeus, St Dionysus the Aeropagite and others as well. He also wrote major essays and studies on St John of the Cross, St Therese of Lisieux, and a number of other Christian saints.
Edward Schibeccxx wrote a number of interesting theological works, including a major study of Jesus in the New Testament.
Cardinal Ratzinger also wrote some interesting theological works (along with Kung he was a Tubingen Professor) though a lot of his material is pastoral in nature, during his tenure at the Congregation for Faith.
Bernard Lonergan is a famous Jesuit theologian who examines St Thomas Aquinas and also the process of conversion, and is useful to study.
Karl Rahner wrote a number of works which analyse theology and conversion from an existentialist perspective, informed by Martin Heidigger’s philosophy in particular.
Walter Kasper, also a Cardinal, wrote some interesting works thought not matching Balthasar in depth and insight.
John Paul II wrote a number of interesting works, including a major study of the mysticism of St John of the Cross. His spirituality is deeply informed by the Carmelite notion of contemplative prayer.
In the mystical/conteplative side of things, I’d rank Thomas Merton as the major monastic theologian and contemplative writer for the 20th century.
Louis Bouyer and Henri Lubac were also great Patristic scholars and theologians (Henri de Lubac had a formative influence on Balthasar as well).