There are many famous world stamps, but some of the most well known are:

British Guiana 1c Magenta (1856): Known as the world’s rarest and most expensive stamp, with only one known copy in existence.  It does not have a fixed catalog price. Instead, its current market value is entirely driven by collector demand, though experts generally value it anywhere between $8 million and $15 million if it were to go back on the auction block. It last sold for over $8,000,000 in 2021
Inverted Jenny (1918): A famous United States error stamp showing a Curtiss JN-4 biplane printed upside-down, from a single sheet sold by a post office in Washington DC.  Only 100 of these stamps are known to have been issued and almost all are accounted for – only position #66 is still missing!  
Mauritius “Post Office” (1847): Famous early colonial stamps issued by Mauritius that are major prizes for collectors.  Only 500 red and 500 blue stamp were printed, and only about 27 to 30 copies of these original 1847 stamps survive today, which were engraved with an error reading “Post Office” instead of “Post Paid”.