This is a difficult set of facts to ascertain. Never can tell when someone is going to pull out a whole collection of rare or one of kind cars. Also, this article can only deal with cars that were actually built by car manufactures, after all aftermarket customized cars are, by their nature rare or one of a kind.
The Helica, is a two seat roadster that has a huge fan drive on the front. Some 30 were built between 1913 and 1926, by Frenchman Marcel Leyat, of which two are known to have survived.
The Rolls-Royce 15 hp, built in 1904 as a result of a new partnership between Charles Rolls and Henry Royce. If featured a new 3 cylinder engine that drove the car at the breakneck speed of 39 miles per hour (MPH). Only six were ever made of which only a single copy is still known to exist.
The 1948 Tuckers, only 51 of these visionary vehicles were ever produced, and all are known by their manufacture serial numbers, ie numbers 1001 to 1051. The development of this car is the subject of the 1988 movie: “Tucker: The Man and His Dream”. Some of these cars do pop up at very high end car auctions. For example Tucker 1038 sold for just over one million dollars in 2009.
The Jaguar D-Type, XKSS version, originally designed as a race car, when Jaguar withdrew from racing, the company modified the car to make it street legal; they added a passenger seat and door as well as full length windshield and ragtop. A fire in the factory destroyed 9 of the twenty-five built leaving 16 to ever leave the factory.
The Aston Martin DB1 2-Litre Sport was made from1948 to 1950. First shown at the London car show of 1948, only 15 ever sold.
The Ferrari 250 GTO, two different versions of this car were made from 1962 to 1964. The Series I and the Series II have slightly different body styles. Only 39 of both styles together were ever made. To buy one the buyer had to get the personal approval of Enzo Ferrari, the company president and founder. In 2010 one sold at auction for more than 17 million dollars.
“The Porsche 916 was never officially produced, but 11 were made in 1972 as prototypes or pre-production models. The 916 has a Porsche 914 body with flared fenders and 15 inch wheels. Also a steel roof was added. Only one was ever shipped to Florida, this copy was the only one that had Air conditioning added.