He was, but Hearns came along 3 decades later. For the 1940s, a 5’11 welter - as SRR was - was crazy.
This is a difficult question to answer because there's no easy way to determine how much was a "gift" and how much was training, acquired skill, and hard work. You'd have to look for outliers like Carnera who just plain shouldn't look or perform like they do, relative to their own era and the training resources available. (Carnera being an example because he's a 6'5" man with 260 pounds of muscle -- very abnormal even today -- and good stamina in an era before PEDs.) Or people with verifiable testing showing them to be extreme outliers from a young age, physically. Or medical exams showing actual advantageous conditions, etc.
In my opinion, it's nobody mentioned here. Natural great athletically co-ordinated fighters such as SRR, Ali, SRL, RJJ, Sweet Pea, Floyd, Benitez, Hearns, Duran etc. are all worthy of mention. Going left field, there are fighters that have had incredible grappling or wrestling/Muay Thai/MMA careers and have then had excellent boxing success or vice versa - amongst this group likely lies my answer.
At 6'3" 210 - 220 pounds and moved faster than most Middleweights in history, balance, reflexes, intuition, and physically stronger than most heavyweights . Not to mention much tougher. How can it NOT be Ali. Especially when considering the size difference between him and most of the fighters he's compared to.
