I don't know if Karrin Allyson is the best singer I've ever heard, but she's damn close. And I can honestly say she's the best scat singer of all time. Ella was always considered the best at scat, but she could never do what Karrin can do. Just find and listen to Karrin's version of "Parisian Thoroughfare." Her mouth is going about 150 mph, matching the speed of the band behind her. And listen to what she does with "The Little Boat" ("O Barquinho").
Amazing what one can find on YouTube in terms of music videos. There must be five or six more featuring Ms Allyson. An exceptional one is a duet with another jazz scat singer almost no one is familiar with, Nancy King. Give a listen to "O Pato" (The Duck).