1926-S Buffalo Nickel
Discounting only the overdates and varieties such as the 1918/7-D and 1916 Doubled Die Obverse, the 1926-S is the rarest Buffalo nickel in Mint State. It has the lowest mintage of any circulation strike delivery of the type with only 970,000 coins produced. Even low-grade Uncirculated survivors are very scarce, while Gems are "nearly unknown" per David W. Lange (The Complete Guide to Buffalo Nickels, Second Edition, 2000).
The example to the left was sold by Stack's Bowers Galleries in the March 2006 Auction, where it realized $29,900.