As is usual, the weigh in rules are different. Golovkin comes in significantly heavier. I was doing a side to side comparison of GGG and Hagler a while back. Golovkin seemed like a much heavier puncher to me, whereas I was a little shocked at how much faster Hagler was. But when you consider that you are actually comparing guys with at least 10lbs weight difference if not more, it all starts to make sense. Not sure I'd favour any of them. Leonard to me would be probably the best candidate to do the job with a stick and move routine on GGG. Even here, though, Golovkin has immaculate footwork and always seems to be able to keep the fight at the distance he wants. Duran has superlative skills in the pocket but I think the size is going tell heavily against him.
Leonard has the best chance here imo, we have seen GGG struggle with fast hands multiple times and I don't think a combination of that + even faster legs and movement would be a fun time for GGG. Plus Leonard was extremely tough. Hearns probably gets walked down with the jab and slept late with a huge overhand. Duran is gonna be deadly on the inside but GGG is pretty safe as long as he stays at range and uses his jab/size.
I thought Golovkin probably could've bullied Duran a good time,but I believe that it's going to be Duran vs Barkley II,where Duran shows his experience and wits to come up in response to the big man's size, strength and the overall gaps in physicality. Duran's winning ratio is still quite small with his size,but I think he could've survive the clinch a bit to a decision.