You have a clear sense of the problem of defining an album from the 1940's. Dustbowl Ballads (vols. 1 & 2) are certainly candidates for the best. And there's no question that "Black, Brown and Beige" is a masterwork of American composition.

But I would argue for including Merle Travis' "Songs of the Hills" (1947). It's an album (four 78 rpm discs in a single binder) and it has a more central theme than Guthrie's (or the other albums listed here).

When we get to the 50's, it gets really complicated.