From Ligonier’s Theologian for the Ages:
John Calvin (1509–1564) is easily the most important Protestant theologian of all time and remains one of the truly great men who have lived. A world-class theologian, a renowned teacher, an ecclesiastical statesman, and a valiant Reformer, Calvin is seen by many as the greatest influence on the church since the first century. Apart from the biblical authors themselves, Calvin stands as the most influential minister of the Word the world has ever seen. Philip Melanchthon revered him as the most able interpreter of Scripture in the church, and therefore labeled him simply “the theologian.” And Charles Spurgeon said that Calvin “propounded truth more clearly than any other man that ever breathed, knew more of Scripture, and explained it more clearly.”
Let’s not forget how Melanchthon changed his views concerning predestination (scholasticism).