As of my knowledge cutoff in 2021, the most expensive currency in the world was the Kuwaiti Dinar (KWD). As of March 2023, it is still one of the highest-valued currencies in the world, with an exchange rate of approximately 3.29 USD per KWD. Other currencies that are also considered highly valuable include the Bahraini Dinar (BHD), the Omani Rial (OMR), and the Jordanian Dinar (JOD). However, currency values can fluctuate over time, so it's important to check current exchange rates for the most up-to-date information.