I was dating a girl who was a supervisor for B of A and I asked her about counterfit money. I always thought that the $20.00 was the number one bill counterfitted but she said no it’s the $100.00.
Domestically, $20 could make the best candidate, but internationally, $100 is the overwhelming choice. Ask N. Koreans.:-)
I used to work for the company that makes the US currency paper, this girl is right. However there are some caveats.
Outside the US most of the counterfited currency is in denominations of 100.00$ US, in the US however the bills tend to be smaller, 5-10$ US.
There used to be an issue with 50.00$ US Bills being counterfited, however with those not in circulation as much they are easier to identify.
Outside the US you are more likely to run into counterfit bills than in the US. Still the Treasury reports total global counterfits, if I remember correctly, at being less than 1% of total outstanding currency.