Harry used the spell Expelliarmus to defeat Voldemort. This spell is also known as the Disarming Charm and forces the subject to drop whatever they are holding. In the final battle, Harry used his signature spell with defiance and it ultimately led to Voldemorts defeat. It is important to note that it was not Harrys spell that killed Voldemort, but rather the fact that Voldemorts own Killing Curse backfired on him due to the Elder Wand recognizing Harry as its true owner.