What's the most exotic item in your numismatic collection?
In terms of items that fall strictly within the parameters of the US Coin Forum, it would probably be my Lesher Referendum Dollar.
Lesher was a pioneer in Colorado. Among other vocations, he was a silver miner. He thought silver was trading below at its then roughly 65 cents per ounce value. So around the turn of the century, he began minting his own large silver coins, hoping to spur interest and business for the silver mines in the region. His earlier pieces were a full ounce, which he expected to trade at $1.25. The pieces made a year later were the same weight as a US silver dollar and bore a $1 valuation. He arranged for various businesses to exchange his pieces, the recipients of whom could either accept or reject the pieces as they wished (the "referendum" part of their name). He even allowed people to redeem the pieces at the local Victor, Colorado bank for "lawful" money in the same way a check was cashed. Interestingly, silver wouldn't reach his $1.25 target until 45 years after his death. Perhaps even more interestingly: none of the approximately 2,000 Lesher Referendum Dollars thought to have been made were ever redeemed.
But from my collection as a whole, it would probably be this large handmade, hand stamped woodblock print 100 Srang note from Tibet during WWII. The rough cut paper and colors are amazing, and it contains dozens of examples of Tibetan religious iconography. From Numistma:
Two holy men are seated under a lime tree. Two cranes, symbolizing longevity, and two deer symbolizing prosperity, lie on the ground in front of the Lamas. One holy man holds a magic bottle, which symbolizes fertilizing the Earth. Above are two flying bats, symbolizing felicity and fortune.
--Severian the Lame