According to Google…and of course it’s the Penny Black, issued on 1 May 1840 in the UK, featuring the profile of Queen Victoria.
The initial print run was fairly large (68,808,000 stamps), so many specimens of the Penny Black have survived. Not so for the world’s rarest postage stamp, the British Guiana 1c magenta, of which only one specimen is known. It sold at auction on 17 June 2014 for US$9,480,00 - the world record for a single stamp auction price.