Number 13 and 17
The number 13 was not used, as per racing tradition. In fact, only one race pre-2014 featured a number 13 car starting in Formula One history: the 1963 Mexican Grand Prix. It was also used in the 1976 British Grand Prix, but was not qualified. As a result of this, the number 14 was often used by a one-car team or a third driver. In 2014, the number was selected by Pastor Maldonado.
The number 0 has been used on a few occaisions, mainly by Damon Hill in 1993 and 1994 after Nigel Mansell and Alain Prost, the 1992 and 1993 World Champions, left the sport. It will most likely never see any post-2013 use.
The number 6 has been used the most, with 931 races. The highest number is active use is number 8, used in 922 races. The highest number is 208, in a Radio Luxembourg-sponsored car for Lella Lombardi at the 1974 British Grand Prix. She did not qualify. It was also used in five non-championship races.
After the death of Jules Bianchi, the FIA decided to permanently retire the number 17 he drove with before his accident.