I would certainly agree that Newton built his success upon the earlier work of others, like Copernicus, Brahe, Kepler, and Galileo. I would not necessarily agree that Newton's Law of Universal Gravitation is man's greatest discovery. Newton understood the limitations of his work in that discipline better than most, and he knew very well that his equations provided a tool to predict motion in a gravitational field, but did little to advance the understanding of how it works. Improved understanding and a better set of tools arrived 200 years later with the work of Dr Einstein, who stood on the shoulders of Newton and others.
Newton and Einstein stride like giants across the landscape of modern scientific thought. It is difficult to mention one without invoking the other in any discussion of modern physics. Personally, I think Newton's primary contributions came in the areas of mechanics and calculus, both of which he essentially invented (with a little help from his friends). His efforts in the area of gravitation were brilliant, but limited by the difficulties he faced in terms of supporting knowledge in 17th and 18th century Europe. He could only go so far.