Q. Which of the following is correct or preferred? I’m guessing it’s the first option. I’m working on a very important, time-sensitive document, and everything has to be correct according to CMOS.

She’s number one in my book.
She’s number 1 in my book.
She’s No. 1 in my book.
She’s no. 1 in my book.

A. All of those stylings are widely accepted. The default Chicago style for numbers one through ten is to spell them out, so “number one” works well. In certain contexts (such as referring to a list), you might opt for “number 1,” based on Chicago style for “page 1”, “table 1,” and other such expressions.