Which are the most beautiful mathematical or physical equations?

I am interested in the elegance of the equation itself (not its visualization, e.g. the Mandelbrot set). Yeah, I know that there is a difference in opinions, but I hope there will be some correlation among experts. I would like to have, let's say, 5 candidates.

This is a specific example, and a very good candidate, IMHO:

e^(i π) + 1 = 0

I would like to have about 5 equations, not just one, so even if you agree with my choice, please post even the equations that seem a bit less impressive, but they still have some kind of beauty in them.

Okay, just to give you another anchor, this one is nice too, but much more simple:

a^2 - b^2 = (a + b).(a - b)