Best Equations

In the spirit of the best constants thread, post what you think is the best (most beautiful, useful, elegant, etc) equation from math and physics.

My suggestion: Euler's formula:

e^(ix) = cos(x) + i sin(x)

especially because when x=π, e^(iπ) + 1 = 0 (Euler's identity)

>>13
Honestly, you really do have to give it up the place where it all started.

However, I think Euler's takes the cake for pure beauty.

Of course another favorite ∫e^x=(lol immature humor)