Who is Frank Sinatra Speaking Of?

I've not been able to find reference to who he was talking about in that instance. But I believe the general consensus is that Ella Fitzgerald was his favorite.

Though I didn't find the exact quote, Frank Sinatra had a great regard for the song and style of Ella Fitzgerald, whom he toured once with. As well he had wanted her to play in his final album DUETS but by then she was too ill.

Some people think it's Jan Daley.

My daughter is a big jazz fan and she also thinks Sinatra may have been referring to Jan Daley.

Here is a quote from a Jan Daley posting:

The major turn in Daley's career occurred when Frank Sinatra caught one of her shows and commented to his industry friends with him, "this girl has got it!"