Who delivered the final blow?

[Eomer of the Rohirrim]

What if Sauron accidentally dropped the Ring—would that have killed him? Busting out the reductio ad absurdum...

Take this quote from Johnny T. himself.

"Gilgalad and Elendil are slain in the act of slaying Sauron." (from a letter)

Admittedly, this does not necessarily entail, but it certainly does imply that Gilgalad and Elendil slew Sauron. The way it's worded does suggest this.

The final blow I take to mean that blow which finally broke Sauron's body.

Oh, and by the way: I'm just guessing that the Kings stabbed Sauron all over his body. It's true that there is no explicit textual evidence to support that but I liked the phrase so that's why I typed it.

[Lindolirian]

For the argument that Isildur cut the Ring from Sauron's hand after he had been vanquished, I found a quote of Elrond's (an eye-witness, you know... very useful)

Isildur took it... But few marked what Isildur did.

I have a feeling that in a campaign to destroy the Enemy of all the Free Peoples of Middle Earth, the completion of such a huge undertaking would be pretty well marked by more than a few. My point is that in taking the Ring, Isildur did not do anything spectacular, but was being discrete after the battle was already won and snatched it from a dead hand.
Now to decide between Elendil and Gil-Galad is a tougher choice. Al I can come up with is that Tolkien keeps mentioning Elendil last... of course that really could mean nothing at all, but in the event of a toss-up, I'll let it decide for me.

[Farael]

I must say I am proud of always being the first one to admit I'm wrong when proven wrong... problem is, it usually takes A LOT to have me accept that it has been proven that I was mistaken

Anyway, I was re-reading Lord of The Rings and I got to the part in which they are in Rivendel and Elrond tells the story of how Isidur got The Ring. And the way he tells it, it does have me thinking that he took it from the dead or disabled body of Sauron. I should also take a look at The Silmarillion yet I'm not in the mood for re-reading it at the moment.

But yes, I must say that it seems that I was mistaken and Sauron was already down when Isidur took the ring.

[Eomer of the Rohirrim]

Well, it's by no means definite Farael but we certainly had some good debate.

On a related point, I think I once voted quickly on one of these polls and then changed my mind; of course, you cannot then change your vote.

Polls, eh...

[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.)