Green’s Dictionary of Slang

1. the best, whether of objects or persons.

1942–4		in G. Legman Limerick (1953) 119: There was a young gaucho named Bruno / Who said, ‘Screwing is one thing I do know. / A woman is fine, / And a sheep is divine, / But a llama is Numero Uno.
1963		in E. Cray Erotic Muse (1992) 221: An Argentine gaucho named Bruno / Said, ‘Fucking is one thing I do know. / All women are fine, / And sheep are divine, / But llamas are numero uno’.
1974		(con. 1960s) R. Price Wanderers 93: That was numero uno for me.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 181: The finest outdoor flick of 1983. We’re talking numero uno. We’re talking top of the Joe Bob Briggs charts.
2013		T. Robinson Hard Bounce [ebook] ‘He’s Cade’s numero uno enforcer’.

2. (US) an important person, the boss.

1976		Time 21 June E6: All the Bicentennial rhetoric and campaign jingoism can’t cover up the fact that we’re not Numero Uno.
2001		K. Sampson Outlaws (ms.) 162: Ratter’s fingered myself as Numero Uno on an upcoming beak deal.

3. (orig. US) oneself.

1977		C. McFadden Serial 29: She’d earned the right to just look after Numero Uno.
1990		S. Morgan Homeboy 212: Don’t be fooled [...] he’s only looking out for numero uno.
2015		I. Welsh Decent Ride 317: You broke the contract, mate, so it’s numero uno fae now on.