Best athlete = most dominant with respect to all eras and contexts. I'd argue as great as Babe was, and she was great, she was never more dominant than Biles has been. Yes, she won, but she really didn't accomplish anything in track that was truly memorable. Her golf career was very good, but not really the stuff anyone talks about today. She gets extra points for versatility, but again, the context matters.
Biles is literally in the conversation on just a couple of fingers as the greatest gymnast that has ever lived.
No one else on the list is spoken about in that same kind of breath.