World Coin Rarities?

Basically a boundless question.
A few modern rarities I like:

The 1954 Mexican Hidalgo 5 Peso. Been going straight up as of late, but always a good value. Interest in Mexican numismatics is white hot and this is a modern key date.

The 1948 Canada $1. Big papa. Has doubled in value in the past 5 years. High mint state grades getting increasingly inaccessible.

1985 Ireland 20p. A true mega-rarity and heavily faked, but a coin I've been after for a while, but rarely sold.

Underrated; the 46/47 Italian rarities. Big market and very hard to come by.

Some quite notable United Kingdom rarities:
Penny: 1933 (around 7 known), 1952 (apparently unique), 1954 (apparently unique)
Threepence: 1937 Edward VIII nickel-brass (a few patterns got into circulation); 1945 silver (all apparently melted down, but one or two known to exist)
Shilling: 1952 English reverse - a few believed to exist.
Halfcrown: 1952 (one found in circulation in the 1960s)
Decimal 2 pence 1983 (a rare mule exists with the 'New Pence' reverse used from 1971-81)
Decimal 10 pence 2009 (a rare mule exists with the Lion reverse used from 1992-2008)
Decimal 50 pence 2011, Olympics Swimming - rare variety with lines passing over swimmer's face, from a pattern die that should have been discarded.

The above are all coins of which only a few are known. Other pattern Edward VIII coins exist, and some other coins like the Elizabeth II 1953 gold half-sovereign, sovereign, two and five pounds that were struck in small quantities but never made available to the public.

Great list, Sap!
Some extremely rare 20th-century French coins:
5 centimes: 1914 cupro-nickel type
10 centimes: 1914 nickel type
20 centimes: 1946B
1 Franc 1914C 1943 'Republique Francaise' type (minted in Algeria during Nazi Occupation of France)
2 Francs 1991 coin alignment (most have medal alignment, and were sold in BU sets)
5 Francs 1934 1936 1939 nickel type 1941 Petain type (most destroyed in WW2)
20 Francs 1939 1973 silver 41mm (abandoned in favour of a 50F coin owing to increase in price of silver, but about 20 are believed to exist)

This is great information thanks.

wow, fantastic list.

Maybe tomorrow, I'll come back and compile what has already been listed into one post for easier access as this topic grows.