ichiban, n. — Green’s Dictionary of Slang

ichiban n.

[Jap. ichiban, number one, picked up by US troops serving in the Korean War (1951–3)]

(US) the best.

1900		D. Belasco Madame Butterfly (1928) 16: I tell you, she’s just ‘ichi ban’.
1909		Ade ‘Modern Japanee’ in Verses and Jingles (1911) 25: Brown as a berry, busy as a bee, / Ichi ban! Ichi ban! Japanee.
1951		N.Y. Herald Trib. 16 Dec. II 5: Ichi-ban (Japanese) means No.1, the best, superior.
1954		(con. 1940s) T.A.G. Hungerford Sowers of the Wind 92: Australia cigarette [...] Number one — ichiban.
1956		W. Lasly Turn Tigers Loose 73: Thomas grinned and formed a circle with thumb and forefinger. ‘Ichi-ban, sir. I wrote her a long letter last night.’.