What's a "best boy" in film credits, and why is there never a "best girl"?
The best boy is the gaffer's assistant. The term is an old one, dating back to when film crews were usually male. While best boys are now often women, the title remains the same.
The best boy (of either gender) is sometimes given the more highfalutin—but less cryptic—title of "assistant chief lighting technician."
—The Fact Monster