My favorite theorem is the Pythagorean Theorem: a$ ^2 $+b$ ^2 $=c$ ^2 $, which states that in a right triangle, a and b being the two sides that meet at the right angle, and c being the hypotenuse, the sum of the squares of the two sides is equal to the square of the hypotenuse. This is a very common but a significant formula and that is why I like it. Other people listed this theorem as their favorite too.
