The British Guiana 1¢ magenta is considered as the rarest post stamp that exists. Produced in 1856 and sold for 1cent this rare piece is now worth around 935.000USD.

It is printed black on magenta paper, and it features a sailing ship along with the colony’s motto “Damus Petimus Que Vicissim” (we give and expect in return). I have also learned that this particular stamp has an interesting story, perhaps you can read more about it on the wiki, as I did.

Untill then Damus Petimus Que Vicissim