To choose only one is very difficult, some time ago I have made the same question but suggesting a list of ten. There are interesting answers and the link is
The number one for me is Newton because he was the first to introduce gravitation and solve the celestial mechanics in a rigurous form, besides many different important issues in Modern Physics. As you can see Richard Feynman is not in my list because I consider that his contributions to Physics are not so important, for instance Schwinger has made a wonderful in QED and Tomonaga too, sharing the some Nobel Prize.
That is a very big question :). Not easy to determine one scientist.However, I would choose Leonard Euiler as the greatest one, for his great contributions to many fields in mathematics, that servers all sciences indeed.