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The correct answer is the British Guiana 1 cent black/magenta ( SG 23 ) of which only one example exists. Last on the market in 1980. See stanley gibbons Commonwealth & British Empire Stamps 1840-1970 on page 112.
If only one stamp exists and this makes it extremely rare then one could also look at Great britain sg 266a ( 2 penny Tyrian Plum ) of which only one example exists and this I think is in the Queen's collection.
Even if he were to sell it on BoB you and I certainly could not scrape the money together to buy it, but sadly you have been knocked off your first place pinnicale. We would have to really find a stinker of a question to try to get you back to top of the class.