1936 Mercury Dime
Only 4,130 Proofs of this date were produced, the first such coinage in the denomination since 1915. It is also the scarcest in numismatic hands with not only the lowest Proof mintage in the series but the fewest certified examples as well. While available fairly easily through PR-67, it becomes quite tough above that grade with PR-68s rare.
The example to the left was sold by Stack's Bowers Galleries in the January 2011 Tampa Bay Auction, where it realized $4,600.