number one

The best, the primo, the absolute highest crowning achievement of any particular quality or invention.

See The Cage (TOS)

Played by Majel Barrett, but credited under the name Majel Lee Hudec in the pilot episode. First officer of the USS Enterprise under Captain Christopher Pike. We never learned her name – she was only referred to as "Number One" – which led to all sorts of fan speculation. Her reserved nature caused some to think that, like the Vulcans, she was from a race that suppressed their emotions, but I never bought that. She was just a tough, no-nonsense officer, a rarity from a woman in 1960s sci-fi. At the climax of the pilot, Number One sets her weapon on overload in a suicidial maneuver to prevent the Talosians from using her and Captain Pike to breed a race of slaves. This woman does not fuck around. But she was not destined to stay around, as most of the crew of the pilot episode was abandoned, and Majel Barrett was given a short skirt and mooned after Spock for three seasons as Nurse Chapel. Sigh. We wouldn’t see that kind of asskicking from a woman again on Star Trek until Tasha Yar.

In the television series Star Trek: The Next Generation, Captain Jean-Luc Picard (Patrick Stewart) often referred to Commander William Thomas Riker (Jonathan Frakes), the second-in-command of the ship, as "number one" due to Riker being the first officer. This may be the reason Majel Barrett's character in the original series' pilot was called "number one" as well.

Calling his first officer this was ingrained into Picard's routine enough that when he was assimilated into the Borg collective as Locutus, he still referred to Riker as "number one" during a communication between the Enterprise and the Borg cube.