For me it has to be Indian lakhs and crores. Try as I did, when in India it still took me ages to work out quantities when those two were in play.So what is the weirdest units anyone has worked with?
In my first job they were "going metric" and I had to convert a load of stuff from "gallons per cwt" to "000s litres per tonne". This was called the yield factor and concerned how much cheese was produced from quantities of milk. Yes, in my first job I was a human spreadsheet!
Also the Swedish mile. But at roughly 10 kilometres that is easier to get your head round.