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.