Mine is the Pythagorean Theorem because of its history and use in today's world.
$ a^2+b^2=c^2 $
In The Thirteen Books of Euclid's Elements, the entire book is pull of proofs that leads up to this great theorem. I find it amazing how long it must have taken to discover that this theorem existed. Also, I cannot count how many times I have written this formula down to solve problems. Not having this theorem to solve problems in math and physics would almost be impossible!
