When listing top female singers for each decade between 1920 and 1969, two considerations must be kept in mind:
1) "Top" singer may refer to most records sold, or it may refer to the far more subjective assessment of "best" singer, on which there may be disagreements; and
2) Different music genres force unfair comparisons between singers of different types of music. A top opera singer, for example, cannot be expected to compete for top vocalist with a jazz, pop, or country western vocalist.
Another factor that is relevant is that some individual female singers were active performers in more than one decade. For purposes of discussion, however, a different singer will be listed for each period of time.
Because of these issues, what follows represents only this particular educator's assessment of best female singer for each of the decades specified.
The 1920s were, as much an anything, a decade of jazz and blues. The top female jazz and blues singer for that period, then, would have to be Bessie Smith (1894-1937), known as the Empress of the Blues.
The 1930s, like the 1920s, were dominated by the jazz and blues performers in areas like Harlem, New York; Kansas City, Chicago, and New Orleans. The top female singer of this period could be Kate Smith, Judy Garland, or Ella Fitzgerald, but Billie Holiday (1915-1959) gets the nod as top female singer of the 1930s.
The 1940s could be the decade of Judy Garland (1922-1969). While "The Wizard of Oz" was released in 1939, Garland's recording of "Somewhere Over the Rainbow" was hugely popular throughout the 1940s, as were her musical performances in films like "Easter Parade" and "Meet Me in St. Louis." Special mention here could go to Dinah Shore.
The 1950s is a particularly difficult decade to select a single "top" female singer, as Doris Day, Rosemary Clooney, Patti Page, Sarah Vaughn, Dinah Shore, and many others were all active during this period. In addition, Billie Holiday was still active, and still great, as was Ella Fitzgerald. Page probably had the most commercial success, or, at least as much as any female singer of the period. Rosemary Clooney (1928-2002), however, gets the nod for this decade.
The 1960s, of course, witnessed a particularly dramatic transformation in music, as rock and roll, already popular during the 1950s courtesy of male performers like Elvis Presley, was making major inroads culturally. Plus, the political and social turbulence of the period resulted in protest music that represented a vastly new genre. Having said that, the top female singer of the 1960s was probably Dionne Warwick (1940-), who wins out over Aretha Franklin, Tina Turner, Janis Joplin, and Dusty Springfield. Warwick's popularity certainly led to commercial success with hits like "Do You Know the Way to Santa Fe," "Walk on By," "I Say a Little Prayer," and "I'll Never Fall in Love Again," but she also enjoyed critical success.