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)