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)

No, that's only in the sense that .9999... is so close to 1 that it is one.
Yes, I know .3333 repeating is roughly equivalent to 1/3, and 1/3 * 3 = 1, but that does not mean .9999 repeating equals 1. You are just saying that it is so close to 1 that it might as well be one.

You see, .333 repeating =/= 1/3. .3333 repeating is just infinately close to 1/3.