It would be iron.
Though the practice of awarding meritorious achievement in any field with gold medallions dates only to the 18th century (the Greek Olympians contented themselves with olive crowns and free meals for life), the ranking of metals comes from an old Platonic myth about the origin of mankind.
The first people who walked the Earth did so in the Age of Gold, which was named because it was rarified, lustrous, and flawless, since gold was well-known since antiquity for never tarnishing. After the Age of Gold came the Age of Silver, which was only slightly less rare and lustrous, although it did tarnish. Eventually the Age of Silver led into the Age of Bronze, which had some lustre, although it wasn't particularly rare and tarnished fairly easily. Lastly came our own time, called the Age of Iron, because forged iron is a dull, grimy black color that rusts very easily and is extremely common--even more common than the two metals used to make bronze.
The first three were chosen by the first modern Olympic Committee to represent how the top Olympians were physically and spiritually greater than normal humans. A fourth medal-winner, being considered the best of the rest, would be awarded the human metal, iron.