Green’s Dictionary of Slang

take the biscuit v.

also cop the biscuit, sneak..., win..., take the cookie
[the fig. ‘sweetness’ or ‘tastiness’ of the biscuit; var. on take the cake v. (1)]

[late 19C+] to beat all rivals, esp. with the implication that the person, announcement, event etc. is even more startling or appalling than might have been expected.