Looking at 160. I think you'd have to have a fighter who is cool under pressure, possesses a great jab and is as tough as nails. Add a variety of punches, heavy handedness and the ability to adapt under fire. Footwork is essential. He's going to have to walk through fire early on, while still landing his own shots. The second half of the fight he's got to turn up the gas and exploit RJJ's average constitution to break him down. I am looking at very technically complete fighters. Somebody like Hagler for toughness and versatility, or Monzon for his fighting brain and ability to control fights. Golovkin is another technically complete fighter that might have a decent chance. Maybe Steele for sheer speed, power and experience. Robinson also comes to mind, because, well, he's the RJJ of his time. RJJ is one of the few fighters that you could say would have a chance of beating any guy at his prime weight in history - if not being outright favoured to do so.
From your premise, LaMotta would do very well, especially in a 15 rounder. Stalker, skilled, with a terrific left hand, underrated defense and of course toughness.
NO fighter is unbeatable esp between welterweight and LT.Heavy, their is a fighter in history with the perfect style,skill,and determination to defeat anyone between those weights. I can see Hagler do it, if he fought with the aggression he fought with Hearns. Hearns with that reach and jab and the threat of landing that right hand probably would make Jones Jr. Reluctant to engage. A strong aggressive awkward fighters like Lamotta , or Walker with that physical grind it out style could smother Jones Jr . wrestling him, elbow's low blows, kidney punches ,holding and hitting, head butting. All the physical style Jones has never had to deal with. Their 's quite a few fighters in history between those weights that could've given Jones pure Hell, and judging by the fact that when he did slow a little he was getting starched which suggest his chin wasn't great just the caliber of fighters he met in his prime wasn't capable of consistently testing him... Unlike say Ali who proved his toughness after he slowed.
Apart from being able to have landed the perfect shot, It would really have needed a blend of supreme confidence/belief and the perfect style. I think the psychological side of things would have omitted a fair few guys. Even though he didn't do a great deal at MW, I honestly couldn't favour anyone over him at the weight. That's not to say I don't think that no one at MW could have beaten him. I couldn't favour anybody over him at SMW either. I think you've got to be looking at the ATG LHW's.
Yeah, but Roy was at his peak at MW/SMW, and Sonny was a great HW. It goes without saying that the great HW's of the past would have beaten him.
Did he duck them though? I'd have given Benn a 4 round window. Dariusz was easy to hit and marked up.
Guys like Basilio, Graziano and LaMotta wouldn't have gotten the job done. Although they were great fighters, a guy like Basilio would probably be a WW today. Those guys were small in height and reach, and they were weighing in below the limits in the days of same day weigh-ins. Then look at the come forward styles of guys like Graziano and LaMotta. Roy's last fight at the weight was in 1994 when he was at his peak at 25 years of age. He destroyed Thomas Tate before fighting Toney shortly afterwards. And he was weighing in as SMW-LHW on fight night.
Although I'd have favoured Roy over Marvin, I can envisage Marvin beating him. Monzon was an ATG, but from what I've seen of him, Roy's speed would have given him fits. GG is too easy to hit and he had a small reach.
Not many fighters were tougher than Jake. But he was small in height and reach, weighed about 160 on fight night, and he was easy enough to hit. I can't envisage him doing that well to be honest.
Tarver had the size and style to trouble Roy, but as well as that, he also possessed the confidence. One's no good without the other. It would needed to have been a blend of all 3 to have gotten the job done, which is why I'd rule out guys like Calzaghe and Liles. Being a talented southpaw alone isn't enough. I think Tarver would always have given Roy trouble. But yes, Roy clearly wasn't the fighter he'd been previously. It's silly to argue otherwise. Nunn would have given Roy a puzzle to solve.
I guess Greb would be another obvious choice on resume; pity there's no footage to break the fight down with.