The question is subjective… what the heck does “best” mean?
Anyway the music that was most identifiable to the citizens who fought in the war came from the Soviet Union.
Here is one such song of which the struggle to liberate the Soviet Union from the jackboot had a palpable tone. When this song is played everybody stands up out of respect of what this song meant to those who fought.
http://www.youtube.com/watch?v=uUHwbppl00s&feature=related
Germany had alot of propaganda based music and i would rank them very close. “Forward to the East” comes to mind as a tune sung during Barbarossa Campaign.
http://www.youtube.com/watch?v=6lyPvjkXi3Q
UK really produced “coping” songs that carried humor for the troops on the front. USA basically focused on “happy” tunes not really music directed for the war, but enjoyed just the same. Japan made a number of propaganda songs, but these are marginal to listen to.