I voted the Garden of Eden. Here is why.
I was contemplating that expression this morning and began relating it to Satan and the eternal Covenant of Redemption. This covenant for those who may be unfamiliar with it is a covenant because it is between more than one party, eternal because it is a covenant with all three distinct persons of the Godhead, therefore (and according to Scripture)was in effect before the world was made, and in fact was the purpose of the world being made (at least in part).
What we see on every page of our Bible is the plan of redemption, which is also eternal and fully formed and immutable as God is immutable, being played out in time and throughout history.
Since that is the case, it was God's intention for Satan to be in the Garden, and for the serpent to deceive Eve, and Adam to fall. Satan existed already, and he could not do what he did without God's permission (as we see in the first chapters of Job). It was God's plan that through the redemption of fallen, sinful men, by the person and work of the Son, he (God) would ultimately destroy Satan and the death and corruption he brought into creation through men, once and forever. In any case, that is how I see it given the final result. The glory and wisdom and perfection of it all is mind boggling.
So the last nail in the serpents coffin imo was when Adam took a bite of that fruit and God cursed the serpent and drove in the nail. "The seed of the woman will crush your head, and you will bruise his heel." Everything in between that event and Rev 20:10 "and the devil who had deceived them was thrown into the lake of fire and sulfur where the beast and the false prophet were, and they will be tormented day and night forever and ever. 14. Then Death and Hades were thrown into the else of fire. This is the second death, the lake of fire. And if anyone's name was not found written in the book of life, he was thrown into the lake of fire."; everything between Gen 3:15 and this, is the last nail in Satan's coffin being carried out in time, across the vast expanse of history. He was doomed to be destroyed. What is given in the above scripture is Satan's death knell.
If the serpent had not tempted Eve, ate, and gave to Adam who ate, there would have been no last nail. And Christ would not have come as one of us to destroy him.