Mariano Rivera is the only unanimous HoF selection, right? The traditional reason cited was that since nobody in the inaugural class of HoF inductees was elected unanimously, nobody else should either.
In any event, I think Willie Mays is the strongest overall candidate for GOAT baseball player. He could do everything but pitch, and do it at a high level. The only challenger I see to him is Shoehei Ohtani, if Ohtani can put together several more seasons where he is both a good hitter and a good pitcher. Even then, Mays will be far ahead of him in every single offensive statistic and Ohtani's argument will be about versatility.
Another strength: Mays was a nice guy. A lot other contenders for GOAT weren't.