Harry Greb lost two fights in convincing fashion to a Sugar Ray Robinson prototype in Theodore Tiger Flowers, the first Afro-American to win the middleweight championship. I am therefore leaning towards the neotype Sugar Ray Robinson to win by unanimous decision.
Robinson is a boxer-puncher but he could swarm.
Harry Greb, from what we have read, was a swarmer-boxer.
Both were fast as HE11 in their prime.
Robinson was the best at welterweight and one of the best at middleweight some would say the best.
Greb was arguably the best at middleweight and a pretty good light heavyweight.
A great big man beats a great little man.
Great swarmers also beat great boxers. Volume it too chaotic for scientific fighters to control. Robinsons chance would have been to knock Greb out but Greb took heavyweight punches.
At middleweight; I see Greb winning via split decision if the fight is at middleweight with Robinson winning some rounds based on some powerpunches hurting Greb.
At light heavyweight; Greb wouldn't be wobbled and he would have won a one sided decision [the bigger fighters get, the stronger punches they can take].
At welterweight; Harry comes in drained and Robinson kills him.
Robinson was taller and longer but that has nothing to do with bulk.
Great fight both fighters are pretty equal but greb lack power but he can throw 1000 punches a round non stop. Robinson had it all power, speed, footwork, chin you name it. This fight would be a good one with robinson winning by a ud a close one though.
Harry Greb would win at middleweight.