This is a very interesting question, because if you were to ask 10 historians you very well may get 10 different answers. When I was in seminary my historical theology professor was asked this question and he gave this answer: St. Augustine, Thomas Aquinas, Martin Luther and John Calvin. Now some things must be considered when reflecting on this assertion; 1. This answer was coming from a Conservative, Evangelical, Protestant perspective (non Protestants will have a different answer); 2. once a person spends time reading and thinking the thoughts of specific individuals, that individual can and probably will be seen as one of the greatest theologians (even though they are not on this list); 3. this is a matter of opinion so any view, I suppose, is as legitimate as any other view.
As I have studied Church History I tend to agree with the 4 names given above, however, this does not mean the plethora of other theologians are insignificant. When looked at through the lens of time certainly St. Augustine must seen as at least one of the top 5 most influential theologians. Augustine wrote on many topics as he reflected on he pondered the mysteries of God. He wrote on the Trinity, predestination, time, salvation, the person of Jesus, the nature and impact of sin, original sin, and many, many others. I believe one of the major factors of him being ‘history’s greatest theologian’ is his place in time. Augustine lived from AD 354-430, so came before many good thinkers. Augustine paved the way for these issues to be tweaked and modified by brilliant followers simply due to his place in history. He lived early in the history of Christianity, therefore others stood on his shoulders. Augustine did much of the initial heavy lifting which allowed others to think about the assertions made by him and improve on them.
Second in the church history timeline would be Thomas Aquinas. He surely is accepted by both Catholics and Protestants as an extremely significant theologian. Not only did Aquinas boil down arguments for God’s existence into 5 short pithy ideas, but he wrote one of the most comprehensive tomes concerning Christian theology in all of history.
For Protestants next to make this list is Martin Luther. This is obvious if only because of his writings and concerns catching the attention of the masses fueling the discontent toward the Pope and his bishops. On the one hand I try not to make short pithy statements like ‘Martin Luther lit the fire that started the Reformation’ because the story is so much more complex than one statement can cover. On the other hand I strive to simplify statements about people and theology for my students so I do have a tendency to come up with what I call ‘bumper stickers’ for them to comprehend some deep ideas. Luther’s writing expressing his thoughts on the bondage of the will is a significant treatment of the issue of original sin and its ramifications. Also of great value is his tenacity when it comes to textual accuracy. He was unabashedly devoted to proper exegesis.
The final name on this list is that of John Calvin. I believe his impact is due what would later be called Reformed Theology. Many followers of Jesus Christ throughout the entire world in AD 2015 are convinced of many of the tenants of Reformed theology. Millions of believers all over the globe follow the ideas presented by John Calvin (therefore believe he was correct in his interpretation of the Bible), I am convinced this is why Calvin is on the list of most significant theologians in history.