What is the rarest coin?

According to The Richest, the rarest coin is considered to be the 1849 Double-sided Eagle. It was originally created by the United States Mint, in Philadelphia in 1849, right at the start of the California gold rush. Its value is estimated to be around $20 million dollars, as of 2014.

As 1849DoubleEagle.org explains, only a single example of this legendary rare coin is known to exist, although for more than a century, there have been rumors of an additional specimen existing. The coin is part of the National Numismatic Collection of the Smithsonian Institution, and is therefore very unlikely to ever become available to collectors.