In Australia, we've had polymer notes for 20 years, and counterfeiting is a thing of the past, to the point where I was puzzled as to why someone in the US was marking my $50 note during a purchase while visiting. Paper notes feel nice, but polymer lasts longer and are effectively counterfeit-proof.

Prior to this, we had a woeful $100 note, which was black and white with a bit of subtle colour. Pretty, but a dumb idea for currency. It was not unknown for folks to photocopy one, scrunch it up, and wander into a convenience store in the early hours when the staffer was tired...