My personal research has lead me to believe big bands were definately in, Glenn Miller's "In the Mood" setting the tone (in my mind)... and if you wanted something "alternative" then there's pre-war blues from the likes of Robert Johnson and the advent of the boogie-woogie.
Check this website out if you want to know the annual top tens of the 40s:
http://www.tunecharts.com/?1940s-Music
(or any other decade, in fact
)