MY best guess would have to be a 1943 copper penny. But a little more specific question would be helpful. Small cent or large cent??
1909
The rarest and most valuable Lincoln cent that isn't an error is the 1909-S with VDB on it. The rarest and most valuable error Lincoln cent is the 1943 copper penny.
Only the Philadelphia mint made pennies in 1900. There isn't really a rarest.
1909 s vbd
The 1909 S VDB wheat cent is the rarest to date.
1909
A 1909-S VDB penny!
The rarest British Penny would be the 1954 Penny. There is one known to exist, maybe two, but there could be some doubt about the second.
The rarest and most valuable Lincoln cent that isn't an error is the 1909-S with VDB on it. The rarest and most valuable error Lincoln cent is the 1943 copper penny.
The two rarest Indian Head pennies are the 1877 and the 1909-S.
Indian head penny
Indian head penny
If you refer to general circulation Australian Pennies rather than Proof coins, the order runs something like this - 1. 1930 Penny is the rarest 2. 1920 Penny with no dots on the reverse 3. 1925 Penny 4. 1946 Penny
Only the Philadelphia mint made pennies in 1900. There isn't really a rarest.
1909 s vbd
The 1931-S has the lowest mintage of that decade.
The 1909 S VDB wheat cent is the rarest to date.