Who is the greatest physicist of all time?
There have been many great physicists over the years, and while it's not possible to say with certainty who was the 'greatest', Isaac Newton and Albert Einstein are very strong contenders for that title.

Isaac Newton (1642-1746) was an English physicist and mathematician  most famous for inventing calculus, classical mechanics. His three laws of classical mechanics successfully predicted the behavior of solid bodies across all observable scales - from the motion of planets to falling apples. For over 250 years his laws were applied with greater and greater precision, always giving the correct answers.

However, in the late 1800s physicists struggled to reconcile measurements of the speed of light with Newton's laws. This one of the observations that led to Einstein's Special Theory of Relativity.

Albert Einstein (1879-1955) was a German-born physicist who revolutionized our understanding of space, time and matter with his theories of relativity and quantum mechanics. His theories of special and general relativity changed our understanding of space and time and successfully explained the motion of light and the behavior of gravity.  His theories of quantum mechanics likewise revolutionized our understand of the sub-atomic world.

In addition to Newton and Einstein there are many other physicists who made groundbreaking discoveries,  including Galileo Galilei, James Clerk Maxwell,  Marie Curie, Neils Bohr, Richard Feynman, and Stephen Hawking.