Post Office Mauritius, 1847 - It is estimated that fewer than 30 individual copies of these stamps have survived and they are valued at between $600,000 or more depending on the condition of the particular stamp being sold. In 1993 a cover bearing 2 of these stamps sold for a whopping $3.8 million, the highest price ever paid any philatelic item.
Sweden Three Skilling Banco, Yellow Color Error, 1855 - The three skilling stamp (yellow color error) was issued in Sweden in 1855. Due to a printing error, this stamp is printed on yellow colored paper (meant for the eight skilling stamp of the same set) instead of the usual green color (used for the three skilling stamp). One copy of the yellow error variety of this stamp was found in 1885 by a young Swedish boy in his grandfather's collection.It is a one-of -a- kind rarity, as no other copies have been discovered to date. In 1996 the stamp was sold to an anonymous collector for $2.3 million at auction.
U.S. Franklin Z-Grill, 1867 - This stamp is the rarest of all U.S. stamps, as only 2 copies are known to exist. These stamps depict a portrait of Benjamin Franklin and are embossed with a "Z-Grill" - being a pattern of tiny squares embossed into the paper and visible on the back of the stamps. The purpose of the "Z-Grill" was to permit the canceling ink to be absorbed into the stamp paper thus preventing those who wanted to cheat the post from washing out cancellation marks.The use of "Z-Grills" was not found to be practical and the practice was soon discontinued. An 1868 1 cent "Z-Grill" stamp sold for $930,000 in 1988.
British Guiana One Cent Black on Magenta, 1856 - For a very long time, the 1856 one-cent "Black on Magenta" of British Guiana was considered to be the world's rarest and most expensive stamp. Production of these stamps did not last for very long. In 1873, a 12-year local boy discovered an octagon-shaped one cent "Black on Magenta", postmarked April 4, 1856, and bearing the initials "E.D.W" in his family's attic. Over the years it became apparent that this stamp was unique, as no other copy was ever discovered. In 1980 it was auctioned to John Dupont for $935,000.
Hawaiian Missionaries, 1851 - In 1851 Hawaii issued its first stamps. These stamps are now referred to as the "Hawaiian Missionaries" because they were frequently used by American missionaries on the islands to send letters back to the continental United States. The new stamps were printed in Honolulu in three denominations (2 cent, 5 cent, and 13 cent). Because the first "Hawaiian Missionaries" were crudely engraved and printed on thin and poor quality paper, very few of these stamps have survived and they are extreme rarities.
The lowest denomination, the 1851 two cent, is the rarest of the set, with only about 16 copies known to exist today. A 2 cent Missionary is valued at about $760,000 in unused condition and about $225,000 used.
Rare Stamps - World Most Valuable Postage Stamp