This is a most difficult question because it's similar to asking "who is the greatest jazz artist?" Since the designation "greatest" does not apply to ANY artist, it is equally difficult to name all the jazz musicians and singers I'd glady listen to at any given time. A short list would include: All of the Ellington catalog, the bands of Count Basie, Thad Jones & Mel Lewis, the Gerry Mulligan Jazz Band, the Village Vanguard Jazz Band, the Charlie Mingus Band and The Jimmie Lunceford Band. Benny Goodman, Artie Shaw, Stan Kenton ... singers Sinatra, Ella, Billie Holiday, Carmen McRae, Sarah Vaughan, and some of the "new kids on the block" such as Tierney Sutton, Roberta Gamberini, and Karryn Allyson.
And please forgive me this note: A jazz artist is not a "what". A hybrid perhaps, but mostly a recognizable member of the human race.