I'm a big geometry fan so instead of going with the usual Pythagorean Theorem I'm going to say the whole book of Euclid. This collection of many theorems lead ultimately to the Pythagorean Theorem, a huge part of geometry and math in general. Things in geometry that we find easy and trivial like, the angles at the base of a isosceles triangle are equal, were at one point difficult to prove, that is why this collection of theorems goes at the top of my list.
