According to the source of much knowledge and misinformation aka Wikipedia, the world's first decimal currency was the Russian rouble:
Russia converted to a decimal currency under Tsar Peter the Great in 1704, with the ruble being equal to 100 kopeks, thus making the Russian ruble the world's first decimal currency.[2]
And there are effectively no non-decimal currencies left in use:
Today, only two countries have non-decimal currencies: Mauritania, where 1 ouguiya = 5 khoums, and Madagascar, where 1 ariary = 5 iraimbilanja.[1] However, these are only theoretically non-decimal, as in both cases the value of the main unit is so low that the sub-units are too small to be of any practical use and coins of the sub-units are no longer used.
So I wonder which will be the first to reverse the trend?
Other gems: Cyprus decimalized in 1955 on the basis of a subdivision of 1000 rather than 100, and Bermuda managed to introduce a decimal dollar and keep the penny, by setting one dollar at 8 shillings 4 pence = 100 pence. And Nova Scotia's decimal coinage, ordered from Britain in 1860, was lost in the post and didn't turn up till two years later.