[Over at Papal Ponty's there's a fun little thread about the "Best Theologians of All Time". I didn't want to clutter it up with all my own pontificating, so now you have the privilege of yet more direct "bogus" pontificating here at FCA!]
I think there are three categories, or criteria, for greatness that might help parse our listings. First, sheer brilliance. Second, historical impact. Third, "thematic" impact.
Did the theologian say deep things, and say them beautifully? Did he or she open new vistas in our vision of God? Sheer brilliance is why Tertullian (coined "trinitas"), Athanasius (dunked Arius), the Cappadocians (in any order, I guess), Augustine (changed eveything), Maximus (dunked the Monothelites), et al. are awesome theologians. They made theology come alive. They repossessed, and transformed, their language and culture for the glory of God, which is the proper goal of all theology.
What did the theologian's life, and not merely his writings, do for the Church and the world? This is an especially important criterion and one EO readers probably resonate with more than others. I've heard it more than a few times by EOs that, in the East, a true theologian is a "seer of God," one who sees and knows God, not merely one who knows of and writes about God. I tend to agree. Theologians can never examine God without simultaneously offering themselves in mystical freedom to be examined by God in prayer, evangelical love, communal openness and ascesis -- even unto death. Theology is the words about the "word" of God -- the theoi logos [Gk?] -- and the Logos is a person. Theology, therefore, must be as personal as its subject. Ideally, a theologian's actions (of faith) speak louder and longer than his many words.
This is, strangely enough, why I think Isaiah, Jesus, Paul and other biblical authors have hardly been mentioned in this thread. They are somehow more than theologians, and therefore not eligible for being the best theologians. We sense they are not simply theologians, but rather the very stuff of theology! Why, though? Why should listing Jesus, Paul and John as our three top theologians be so coy, so naively "Sunday schoolish"? Why is there a line between these giants of the faith and the giants that followed them? Because of their amazing words and great analogies? Their antiquity? No. For, as St. Paul says, wise words without love are but arid clanging. Love, then, is the greatest mark of the greatest theologians. So, when I refer to a theologian's historical impact, I mean not just how "big" or "heavy" his ideas were/are, but rather how deep and vital and "cruciform" his life was in the mystical communion of the saints. Hence, for example, Our Lady is one of the greatest theologians for the simple reaons that she "pondered these things in her heart" and now sheds her Sophia-wisdom in intercession for us, her "other children."
This second criterion may seem like a cheesy cop-out to some hardcore theology fans -- "Give us dogmatics or give us death!" -- but my point is not to clear the board of these other genuinely great theologians. My goal is simply to set up one criterion among others for what makes a great theologian. Call it the "Best Martyr-Theologian" category. Great theology, in this sense, is not just a question of making theology come alive (as above), but in fact of being theology alive. Hence, in this second category, I think someone like St. Maximus the Confessor, who bore the fruit of his theology in his own mutilated, persecuted body, is a tremendous theologian. His impact -- as a thinker and as a marytr -- is staggering. The same weight of living-theology, I think, goes for St. Augustine, who lived his theology through lifelong repentance and in the vulnerability of monastic community.
Did this theologian teach things about particular truths of the faith unlike anyone else? Did he or she meet certain specific challenges better than others? Some theolgians are just all-around brilliant: inexhaustible, light-giving, life-giving guides we love like parents. But other saints shine most brilliantly in only a few particular areas. For example, St. Francis de Sales (my, ahem, patron saint) may not have been the greatest systematic theologian, but who can deny he was a master of pastoral and moral theology? Or consider the brilliance St. Louis de Montfort on lying, even apart from lacks in other aspects of his theology. Or consider the Desert Fathers; they are outstanding guides for the "inner eye," even without magisterial works of dogmatic theology to their credit. Conversely, St. Athanasius may not have dwelt too much on the moral dimensions of marriage and social justice, but no one denies his impact on the Trinitarian controversies.
To sum up, ranking theologians depends on parsing among 1) how brilliantly theological they were, 2) how dynamically their lives impact life as "living works of theology," and 3) what issues we're discussing for the ranking. In spite of all this, though, you'll notice I refrain from making a list (or lists) of my own. Like being asked on the spot to name your favorite novie of all time, I beg off cuz it's just too difficult to weed the great from the really great! Who are the greatest theologians of all time? Yes!
Finally, while, as some have cautioned, I do think this sort of exercise could devolve into a theological pissing contest (the ecumenical equivalent of Pop-Faithdom's tendency to "claim" celebrities as coreligionists), if we avoid any bickering, I think rankings like these are good to help reacquaint ourselves with the Tradition. Who really are the great Doctors of the Church? When we look back over the long work of God in our world, what figures are great guides for us today? Who are the martyrs we can follow in contemplation and in death? Who are the masters of certain tough issues? Who must we, in all humility, turn to as holy vessels of God's wisdom? These are vital questions, and amount to one other great reason to heed the living, loving wisdom of God which poured from Christ's side into the Tradition, into so many holy lives, and flourishes even into our own times.
 One of the difficulties of ranking theolgoians is that of historical seniority. Like it or not, especially for Cathlics and Orthodox, very often the highest standard is, "the older the better." There's no gainsaying the importance of St. Ignatius or Irenaeus or Justin Martyr for the simple fact that they laid the foundation, set the very parameters, for what all subsequent greats would theologize. All later theologians have little choice but to defer to their ancestors, since the essense of living (verb and adjective) Tradition is to pass one what was passed on (paradosis) to you. So, in this second category, the older really could be the better, I guess; but in the first category, we'd want to compare each theolgian's work in terms of its "immediate" impact upon us today, regardless.
 Nevertheless, without ranking anyone, I'll cave a little and list here a few more contemporary theologians (showing my total Western and Jesuit bias!) I think deserve consideration:
Bernard Lonergan, S.J. -- brilliant syntheses in both secular and more strictly theological domains
Henri de Lubac, S.J. -- ecclesiological genius, with great discussions the Eucharist
Yves Congar, O.P. -- ditto on ecclesiology, with a big splash of pneumatology
Karol Wojtlya -- theology of the body is the theology for the future: moral, existential, relational, pastoral, phenomenological
St. Francis de Sales -- also a great moral guide, esp. about living devotion and knowing your are beloved by God
Dietrich Bonhoeffer -- a modern martyr as well as a brilliant moral theologian
John Henry Cardinal Newman -- wonderufl homilist, excellent ecclesiologist, solid ethicist, esp. about the epistemology of obedience and the incoherence of rationalistic autonomy