Great Britain issued the world’s first stamp, the Penny Black and GB stamps have always been among the world’s favorites. There are many rare Pound value early stamps and many watermark and plate variety rarities. But the rarest regularly issued Great Britain stamp is the two shilling brown of the 1880 issue (Scott #56).
This stamp is rare for two reasons. First, there were one shilling stamps and five shilling stamps already in use at the time and so there really wasn’t much need for a two shilling issue. But second, and more importantly, the color of this stamp made it look very much like the 10 
pence red brown of the same set. This was confusing to both the public and postal workers who came to rely on different colors of stamps to determine that the correct postage was paid. The two shilling was little used and was soon withdrawn.
The two shilling brown catalogs for $3750 inused condition in the current Scott catalog. There are more expensive regularly issued Great Britain stamps, but there are none that are harder to find and of which there are fewer examples still in collectors hands.