Hum tough choices. They all have strengths. My vote goes to Fiona Apple though as I first heard her sing on a CD called 'Tidal'. 'Criminal' would almost be my top pick off the CD but I must confess I like 'Shadowboxer'. I tend to listen to the whole composition when listening to music. I like Fiona's voice.
As a side note I have listened to a lot of different music over my years and a lot of the names of artist are gone, with their music. But I do remember listening to Fiona in the mid 90's and being impressed with her work. Does having thrown out 800 cassette tapes last move, because they were too faded to play anymore. Or currently having around 1000 cd's mean anything? and almost 280 hours of downloaded music on my computers. Yes, I admit.. I like music. Radio tends to be the source of picking up new artist.
For me, there is a definite tie for the best female singer from this list; I choose Whitney Houston and Mariah Carey. Both ladies have an excellent vocal range. I always enjoy listening to their duet "When You Believe." But the songs that make both ladies stand out are Whitney Houston's song (which seems to be a favourite with all the ladies here on the board) "I Will Always Love You" and Mariah Carey's "Vision of Love." When these ladies hit the high notes, you can't help but jump to your feet and applaud!