[Formendacil]

This being a Fordhim-posted poll, I think myself fully justified in answering as pedantically as the question allows.

The final blow sustained by Sauron was clearly that of Isildur. Although I would not call it the killing blow, for Sauron was already incapacitated (in my opinion) and not dead (and would not be for 3000 years), it was the final blow to be struck directly against his body.

While the the attack by Gil-galad and Elendil can be more directly said to have caused his incapacitation, and while it was Gollum's misstep that actually killed him, the final blow landed on Sauron's body, the last physical hit, came from the hilt-shard of Narsil, wielded by Isildur.

(Note: My vote shows my real answer to the spirit, if not the letter, of the questions.)