Excluding the Micro O coins, there is no singular key date of the series, but it is loaded with semi-keys. Most mintmarked coins
from the 1890s are tough to find in any grade above VG (although the low-mintage 1892-O is always available in high grades,
because many nice specimans were saved as the first year of issue). The most difficult coin in the set to find in high
grades is unquestionably the 1904-S, and in true mint state, the coin is even rarer than the 1901-S quarter. Despite
population reports (which contain numerous resubmissions and some overgraded AU and cleaned AU coins), there are
almost certainly fewer than 25 to 30 mint state pieces in existance.
The best single reference on this series is The Complete Guide to Barber Halves, by David Lawrence, with detailed information about the series, and date-by-date analysis. This is an indispensible reference for the Barber Half collector.
Barber Coin Collectors' Society