When i think of least gifted athletically I think awkward. And there was no one more awkward than the Espana brothers. They couldn't box, they didn't throw their punches correctly, they had no defense but rather recoiled in an ungainly way, stiff, no muscle-tone but had long, neanderthal-like arms and were able to put a fair bit of leverage on them. Ernesto and Crisanto have got to be the prime examples. Scartissue
But it's also said in the lore of boxing that the great Charley Goldman "sculpted Marciano" as though the raw material the Rock provided was a slab of marble or clay in the hands of an artist.
Crisanto Espana was almost a world class marathoner. Definitely strong and definitely athletically talented.
And Floyd Mayweather, Jr didn't come out of the womb with all those tricks. He started training at 2 years old. Marciano didn't start boxing until well into his 20's.
Marciano had great power and functional strength mostly from the outset, and training that kind of stamina is hard to believe. Jones and SRL also worked their balls off in the gym. You're right about Pep and Ali though, stories of slouchiness abound.