Every statement of this type needs almost as many asterisks as it has words: Who do we count as an athlete? How do we define best, over what span of time? Pick your definitions and you pick the winner.
I've built and deployed a small dashboard using 2026 results of various athletic and swimming disciplines — the ones where individual performance is mostly measurable and the problem is the comparisons between different events and sports — and used statistical analysis to come up with scores that can be compared across disciplines.
The tool is here (in an early beta version). As of making this post the three best athletes in the world among the more than 10,000 in the data are Gretchen Walsh, Summer Mcintosh, and Ziyi Yan, and Walsh is so far ahead of the rest that she's almost in a category of her own.