Grand slam winner
A grand slam winner is a contestant who becomes an octochamp (or viscount in case of Series 46), series champion and champion of champions. This is regarded as the highest accolade to achieve on Countdown.
Grand slam winners are typically undefeated in all of their televised competitive games alone, excluding games in the Supreme Championship, the 30th Birthday Championship and special episodes (unless they had a previous stint as a contestant in an earlier series, in which they lost a game). As a point of interest, each of these contestants' defeats (if any) are listed here.
To date, there have been six grand slam winners:
- Harvey Freeman won Episodes 459 through 466, the Series 10 finals, and Championship of Champions III. He also won the Supreme Championship. His only loss as a solo player on Countdown was to Mark Nyman in Episode M45; as well as a special episode where his team, captained by Gyles Brandreth, lost to a team captained by Bill Tidy.
- Nic Brown won Episodes 658 through 665, the Series 14 finals, and Championship of Champions IV. He did not return for the Supreme Championship nor participate in any special episodes, and thus remains undefeated.
- Don Reid won Episodes 1361 through 1368, the Series 25 finals, and Championship of Champions VII. His unbeaten record was ended in the Supreme Championship by John Clarke.
- Scott Mearns won Episodes 2553 through 2560, the Series 41 finals, and Championship of Champions X (a shorter-than-normal CoC in which he received a bye to the semi-finals). His unbeaten record came to end in a special episode against Natascha Kearsey.
- Graham Nash won Episodes 2813 through 2820, the Series 43 finals, and Championship of Champions XI. He also won a special episode. He did not return for the 30th Birthday Championship, and thus remains undefeated.
- Zarte Siempre won Episodes 6774 through 6781, the Series 78 finals, and Championship of Champions XV. He previously won 5 heats in Series 69 before losing his 6th, and has more wins than any other grand slam winner (20).
All but one of these were No. 1 seed for their finals; the only exception is Graham Nash, who was No. 2 seed behind David Williams.