Don't know your field... but I'd say Trade Dollars, $3, and $20 are the most commonly counterfeited. Never buy one of those raw unless you know your S%$
NGC did a top 50 list for what they think are the most counterfeited. Surprisingly trade dollars didn't make the top 50 at all and we all know how much they're faked. The list is extremely heavy with gold especially $2.5 Indians. https://www.ngccoin.com/resources/counterfeit-detection/top/united-states/