Green’s Dictionary of Slang

number one adj.

also No 1
[widely popularized after its importation by veterans of the Korean (1950–3) and Vietnam (1964–75) Wars]

(orig. US) first-rate, excellent, important, influential, best.

[US] R.W. Griswold Correspondence (1898) 242: I have some beautiful poems by me by Mrs. Barnes. [...] They are No. 1, full of passionate feeling and eminently worthy of a place .
[US]H.B. Stowe Dred I 324: ‘You got a good cook in your lot, hey?’ ‘Got a prime one [...] a number one cook , and no mistake!’.
[NZ]T. Moser Mahoe Leaves 38: ‘Canton English’ [...] consists in being able to ring all the changes on the words ‘Savey,’ ‘Comprador,’ ‘number one’ and the everlasting ‘pigeon’.
[US]J. O’Connor Wanderings of a Vagabond 217: It is absolutely necessary that the proprietor of these houses should be widely known as a No. 1 sporting man.
Chicago Life 27 Apr. to rent — The elegant, spacious house at No. 84 Fourth Avenue — first class in every respect, and very desirable for a party capable of running a No. 1 place.
[Aus]Bulletin (Sydney) 30 May 22/3: ‘[T]he little woman’ and her attendant sirens were lavishing their sweetest No. 1 smiles upon two sunburnt ‘Jackeroos’ from the Namoi.
[US]W. Norr Stories of Chinatown 75: A ‘hop-toy’ of Lee How’s No. 1 opium.
[US](con. 1875) F.T. Bullen Cruise of the ‘Cachalot’ 215: Samuela [...] very earnestly informed me that he was no end of a ‘number one’ whale slaughterer.
[US]I.L. Nascher Wretches of Povertyville 211: The more ambitious criminals, burglars, forgers, and what are known as ‘big guns’ or ‘number one men,’ do not work or live in Povertyville.
[UK]‘Sax Rohmer’ Dope 116: ‘Hello! hello!’ croaked a harsh voice. ‘Number one p’lice chop, lo! Sin Sin Wa!’.
[US]D. Hammett ‘The First Thin Man’ in Nightmare Town (2001) 391: I hear about you, Mr. Guild. Number-one detective. You catch ’em my brother.
[US]A. Lomax Mister Jelly Roll (1952) 206: He was the Number One hot band then.
[US]Lait & Mortimer USA Confidential 98: Probably the No. 1 Sicilian in all New England, which makes him the No. 1 citizen.
[UK]H. Pinter Caretaker Act II: I’ve got a brother whose a number one decorator.
[US]J. Breslin World of Jimmy Breslin (1968) 151: The kid came over, took a cigarette, and leaned over for a light [...] ‘Number wan.’.
[Aus]A. Buzo Rooted I iii: I’m Simmo’s number one girl.
[US]R. Sabbag Snowblind (1978) 193: This is mah ma-an [...] He is my ace number one brother.
[US]C. Hiaasen Tourist Season (1987) 187: He’s the Number One Nacho.
[US]S.L. Hills Tragic Magic 54: This is my number one man.
[UK]M. Coles More Bible in Cockney 41: You’re gonna make me your number-one guest.

In compounds

number one cell (n.)

(N.Z. prison) a padded strip cell used for mentally unstable prisoners or suicide risks.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 126/2: number one cell n. 1 a padded strip cell designed to hold suicidal inmates.
number one comb (n.)

(N.Z. prison) an extremely through cell search.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 126/2: number one comb n. an extremely thorough cell-check.