that's a tough one!
i'm going to say that its a toss-up between isaac newton, james clerk maxwell, and albert einstein.
isaac newton unified the heavens and the earth under gravity and the fundamental theory of calculus.
maxwell unified electricity and magnetism.
einstein unified electricity and magnetism and gravity.
the next great physicist will hopefully unite E&M, gravity, and the quantum forces.
this is a very hard question though. i could think of so many more people. in short, though, people as a whole have contributed a great amount. its impossible to pinpoint a specific person though!