Best introductory example for learning physics

[Loren Booda]

What specific physics best represents a comprehensive introduction to the overall subject?

For instance, Richard Feynman used to say that the two-slit experiment was most basic for knowing quantum mechanics.

 

[Janitor]

The royal road from Einstein's 1905 Special Theory of Relativity to his 1915 General Theory of Relativity went through the Principle of Equivalence in 1907. Einstein later called this "the happiest moment of my life," so obviously he thought it was a crucial insight in generalizing STR to GTR.