Hardest Country To Identify Stamps From

coastwatcher ,

The most forged country  - not easy based on the type of forgery

The most regular issues forged - probably Spain
My research shows every stamp between 1850 nd 1930 was forged most with several types
These were mainly postal forgeries
The same applies to their colonies

France is a close second - primarily the Ceres & Napoleon issues and ALL the Colonies

And a (dis)honourable mention to Japan. Aside from being rather difficult to ID. the forgeries abound in the early stamps to 1900. Forging stamps was not criminal until 1898. The packet and tourist trade provided lots of opportunities. Fortunately, the actual forgers were not numerous and only a couple of Euro forgers got involved.

The most overall, some ties here

Russia, so many wars and currency changes between 1880 & 1930
Not unusual to find up to 50 different forgeries for a single issue
One of the worst places for overprint forgeries

US locals - every one was forged, most multiple times. The most prolific forger, Samuel Taylor, I have over 1500 of his varieties.
The Confederate cities who issued are also in the same league - no one was checking or cared
These 2 also have a host of bogus/phantasy stamps