My grandson came up with a contender...

that didn't occur to me but I think qualifies... Michael Phelps (sp?).

For what it's worth... I agree with Waterfield that it's an impossible question... and that there is no 'one' GOAT. But I think we all see feats of athleticism that strike us as 'above'... could be any sport.

Personally.. I don't think any athletes impress me more than women gymnasts... what they can do I can't even fantasize about doing. But naming one? Not possible.