I don't really have a favorite theorem but I love anything related to trigonometry. I'd pick Pthyagorean Theorem as one of my favorites because it has something to do with trigonometry. With this theorem, you can find the lengths of each side of a triangle. It can be easily worked with other trig identities too. If you were given a triangle with one given side and two given angles, you can use the basic trig identities to find one more side then use the Pthyagorean theorem to find the third side.
Sarah and Zach chose this theorem as well.
