Artie Levine, Rocky Graziano, Randy Turpin, Terry Downes, and the European guy that could swat, Vanucci. Something like that, who knows really, the only monster puncher he fought was Graziano i'd suggest. Graziano and Levine aside, I think Robinson might tell you the hardest hitter that landed on him was Jimmy McDaniels, although that was gym.
This is a challenging question because Robsinson fought so many guys we've never seen, and many fighters known for being tough (Lamotta, Turpin, Olson, Jones, Basilio, Fullmer, Downes) but not many known primarily for their punching power. Graziano is an obvious pick, however, and Artie Levine, who floored Ray and nearly knocked him out, has to be mentioned although there's no video of him for us to watch, unfortunately. There is a video of Ray talking about getting clocked by Levine and he's almost laughing.
Turpin could punch, took a guy who with a couple shots that took Marciano into deep waters… was Don compromised at the scales? Maybe but RT could definitely SWAT.
There was a Welterweight George Costner who was making noise that claimed the nickname Sugar. They fought but I'm not sure Ray felt his power. Robinson knocked him out in 1 round.
Jean Stock was a good puncher. Knocked out a young Randolph Turpin and even landed a good punch (after getting up from the 1st knockdown) before Sugar Ray knocked him out. This content is protected
well he fought over 200 fights against at least a hundred fighters, so one might have to go back through the annals & records to get this right. I suspect it could well be 4 or 5 others on the list over at least 2 or 3 mentioned, if not all of them... I'm not going digging, however Ray fought TOP fighters for over 20 years, so only RAY and willing researchers know that.
Jean was a disastrous puncher, he was lightheaded, wobbly and completely set up for the vicious ko and he still landed an ARM PUNCH that stopped Ray right on his tracks