The Swiss 1000 Franc note. These are hard to find now, at least in the part of the USA where I am from.
I once owned one. The bill is chock full of interesting anti-counterfeiting features.
From the Wikipedia
article on the Swiss franc:
According to the central banks, the ratio of counterfeited bank notes is about 1 in 100,000 of real bank notes for the Swiss franc, of 1 in 20,000 for the Euro, of 1 in 10,000 for the United States dollar and of 1 in 3,333 for the Pound sterling
Given that the British pound is by far the most counterfeited money among the four major currencies (CHF, EUR, USD and GBP), I strongly advise the Bank of England to leave the Euro alone and finally get themselves busy with strengthening the anti-counterfeiting measures for the sake of the Pound...
Those numbers are quite scary... I thought it would be the 100 dollar bill that was the most counterfeited..