As I've seen in the movie Harry Potter and The Deathly Hallows (part - 2), Voldemort's last spell produced a green light. I think it was Avada Kedavra. On the other hand, Harry's spell which was locked with it, was of reddish color. Harry's spell destroyed Voldemort's body slowly after Voldemort cried for a while.
From what is shown in the movie franchise and documented on the Wikia,
- Avada Kedavra produces a green, not reddish, light.
- Avada Kedavra doesn't destroy the body of the victim.
- Death by Avada Kedavra is instant and painless.
This suggests that Harry's spell which killed Voldemort wasn't Avada Kedavra. Then, what spell was that?