Even as far as females go, Katie Ledecky is better at her sport than Serena at hers. In fact, Katie might be the greatest ever, mens or womens in any sport.
I tend to put Ledecky and Simone Biles as Legends 1 and 1A among the current generation of female athletes with Serena as a solid and firm third place.
When you talk GOAT, it should be someone who smashed and redefined paradigms, and both those women have. Ledecky redefined what a female distance swimmer can do.
Biles has been so amazing for so long, even if the spotlight falls off her outside of Olympic years. She took a year off after Rio, and when she reappeared earlier this year, it was like 'Hello, the rest of the world is competing for second again this year' by the time she stepped up for her second rotation.
Oddly, I have a hard time putting Michael Phelps over Ian Thorpe. Phelps was the greatest Olympian of all time, but he had a lot of ups and downs throughout the quads, and there were years when he was chasing Ian Crocker or Ryan Lochte at Worlds or Pan Pacs rather than consistently coming in ahead of them. (If only Crocker's career hadn't been moderately derailed by mental health issues...) Thorpe in a 400 or 800, like Ledecky, had this sense of inevitability- the only way someone was going to beat him in his core events at his peak was that time he got DQed for falling off the blocks at Aussie Trials.