Langford is too old and too short. Greb would never stand a chance in the classics section. The only person who could hypothetically beat Langford would be Tom Sayers.
The amount of rounds, and ring size can change my pick. Over ten rounds, with 20 x 20 ring, I'd pick Langford via late round TKO. Sam hit too hard, would not be hurt in this match, and had very good offensive skills.
Langford or Harry Greb ? Who truly can tell ? But many, many years ago in an old Ring Magazine Nat Fleischer ,who saw them both and praised them both, was asked the question " how long would it take for Sam Langford to catch up to Harry Greb and ko him "?. Fleischer answered," I don't think Langford could ever catch up to Harry Greb"...For what it's worth...
I go back and forth on this one. Two of my favorites. I think it depends on whether Langford can stop Greb. Even in his prime Langford wasnt knocking out every fighter he faced. Factor in that Greb was incredibly durable and slippery and I would say if it went to a decision Greb would win on points unless he got so hurt he would fight defensive exclusively. If its a stoppage its obviously going to Langford.