Who is the most famous physicist of all times? It can either be the Father of Physics or just the most famous one. The person people think of when they say, "physics."
Newton got everything started. He's like the father of physics. Sadly, he got gravity wrong , which caused him to lose a lot of popularity. I'd have to say Einstein is the most famous, and rightfully so.
His main idea was that things were pulled to the center of gravity. This was a revolutionary idea that worked... terrestrial objects and the such. However, this theory doesn't really work in the grand picture of the universe. If everything moved towards the center, the universe would be much different from how we know it. It's really complicated stuff that the Theory of Relativity talks about. Of course, Einstein's ideas were at the opposite end of the spectrum of Newton. The string theory is something in-between.