1961 Lincoln Continental Convertible SS-100-X
President: John F. Kennedy, Lyndon B. Johnson, Richard Nixon
This is easily the most infamous car in presidential history. It's that car JFK was riding in one day in Dallas, TX at 12:30 on the 22nd of November, 1963. You know the rest.
After Kennedy's assassination, the car was retrofitted with a hard roof, titanium armor, bullet-resistant glass, and solid aluminum rims that ran inside the tires in case of a flat. It was painted black, rather than blue, when it passed to Johnson.