What is the least common US quarter?

I assume you want to know what the rarest quarter is, either in terms of mintage or value.

By mintage, it's a tie for the 1873-CC and 1827 issues with only 4,000 of each struck.

For value, it would almost certainly be a quarter dated 1804 (not to be confused with the legendary 1804 $1 coin) that sells for nearly a half-million in top condition.