What is the most valuable gold coin ever sold?

Could you tell me, what is the most valuable gold coin that has ever been sold at auction or in private sale? I'm particularly interested in knowing its name, the year it was minted, the unique features that made it so valuable, and the staggering price it fetched when it was sold. Additionally, I'm curious to know if there are any other notable gold coins that come close to its value and what sets them apart as well.