France
Louis XIV gold 2 Louis d'Or 1691-& XF Details (Surface Hairlines) NGC, Aix mint, KM280.15, Fr-428, Gad-259. Listed simply are "Rare" in the Standard Catalog of World Coins and similarly treated in Gadoury with only dashes where most issues have prices. In addition to this piece's considerable rarity, it's also rather attractive, and far exceeds the appearance one might expect from the NGC grade. In fact, it's hard to not find the NGC grade incredibly conservative, as the strike is actually rather bold for the type with few remnants of the undertype and actual friction on the surfaces is confined to the highest points, the fields retaining much of their original flash. Extremely trivial hairlines can be detected upon magnified inspection, but otherwise their presence is essentially nonexistent. All considered, a coin that will undoubtedly appeal heavily to the specialist with little to no concessions having to be made on quality as is often the case with extreme rarities.
Estimate: 4000-6000 USD