As of September 2015, Time magazine lists the most valuable collectible American penny as the 1943 copper-alloy cent. In 2015, a dealer in New Jersey sold his 1943 copper-alloy penny for $1.7 million.
In 1943, when pennies were being converted to steel, only 40 copper-alloy pennies were created, most likely by accident. In 1958, the first 1943 copper-alloy penny was sold for $40,000. In 1996, another dealer sold the rare penny for $82,500. The high collection value of this penny makes it a target for counterfeiters; many fakes are made by coating the steel cents with copper, or by altering the dates of other pennies made in the 1940s. To tell if the 1943 copper penny is genuine, use a magnet; if the penny sticks, it's not copper and therefore is a fake.
TheFunTimesGuide.com gives a general list of pennies that are worth keeping, based on the year. All Lincoln wheat pennies made from 1909 to 1958, Lincoln Memorial pennies made from 1959 to 1981, and copper Lincoln Memorial pennies made in 1982 make the list of pennies that are worth between $1 and $100,000, depending on the specific year, and condition of the coin. The most current valuable penny on the FunTimesGuide list is the 1999 copper wide “AM” penny, which is valued at $500.