It depends on your definition of beauty.
Can somebody explain the most beautiful equation in math thanks
A = π r2 is beautiful, but trying to visualize irrational pi is troublesome.
e = mc2 is beautiful, but it's much too difficult for somebody like me.
My vote has to go for c2 = a2 + b2
It's easy to understand. Draw a right triangle, then draw a square onto each side.
Visualize the area of the square on the hypotenuse equal to the sum of the areas
of the other two sides. Simple and elegant. Thanks, Pythagoras, I owe you one.