The general consensus on ESB is that Oscar De La Hoya was better than his protege Canelo, so how would Oscar in his Prime have done v GGG? I could see Oscar walking GGG down like Canelo did, but going one better and stopping GGG late on.
Oscar in his prime was a great offensive fighter, no doubt, but he was never a full fledged Middleweight. In fact, I'm pretty sure his fights with Sturm and Hopkins were under the 160 lb limit (I know for sure the BHOp fight was at a catchweight.) While I'm also under the belief that Oscar was more talented than Alvarez (although I think Canelo might be the superior defensive fighter,) he did come up from 130 lbs whereas Canelo started at WW as a teenager. Alvarez' frame allows him to comfortably make MW now, in fact he looks like a stocky MW just a bit undersized for the division. Oscar was never a true MW. Because of that reason, Oscar would more than likely get knocked out vs a prime Golovkin. Oscar has very good technical ability and can cause Gennady some issues if his movement is on point. The problem is Oscar will eventually engage gennady at some point in the fight and when it turns into more of a phone booth scrap, that's when he'll get knocked out by something. I've seen GGG drop full size MW's with a jab, I have no doubt he'd knock Oscar out. Maybe another liver shot a la Hopkins.
Oscar too small. Welter version vs Canelo or GGG Oscar in his prime would humiliate. He was a great fighter but I loath him as a promoter and will never buy a golden boy fight again.