JMM best attribute was definitely his timing and counterpunching but it is painfully obvious that Duran was the better overall package and I am not actually sure that JMM had better timing; they both had amazing timing TBH.
Duran is the better greater fighter. I do believe marquez had better offensive timing though. Durans timing looked great against stationary middleweights, junior middleweights. Against welters and lightweights his timing was still good (As seen in dejesus 3) but alot of time he relied on his savage intensity, strength and power to just beat those guys down. Marquez due to his very average footspeed, had to become an expert in suckering guys into big shots, on the way in or on the way back out of an exchange.
I think Roberto Duran vs Julio Cesar Chavez at either 135 or 140 would have been an outstanding display of toughness on both sides.
I've always thought Chavez Duran would be a violent fight and short, they'd meet in the center of the ring and brawl . Chavez has to come forward and he has zero physical advantages over Duran. Roberto is a better boxer and is better defensively and would nail Chavez coming in resulting in big exchanges. Once JCC can't come forward he's a target . Duran stops him in 5 .