Her albums with Louis Armstrong are a great entry point. Simply some of the most effervescent and delightful singing ever put to record. The way they play their voices off one another, the way they seamlessly slide into characters and give each song touches of themselves, is the work of real craft.
Why does an entry point to Ella need assistance from Louis Armstrong? His vocal instrument was unique, peculiar and spectacular. While they could certainly play off each other, his attempts at vocally harmonizing _with_ Ella are mostly cringeworthy musically. Rather than recordings where she had to adjust to his limitations (like letting him choose the key), why not instead listen to any of her highly-acclaimed "songbook" recordings?
This piece is a good advert for the book it's reviewing. I had no idea the critics thought poorly of her in her time - how could anyone listen and not think she's sublime!
You can't really have a GOAT in a creative field -- I would never ever want to diminish Nina Simone, Billie Holiday, Sarah Vaughan -- but really Ella is the GOAT. She just makes perfection sound so easy.
Listen to the live recording "12 Nights In Hollywood" and you can hear that not only is she a beautiful singer, she's an amazing entertainer who came up in a very tough school.