My previous response to a similiar thread:
I have always maintained that Dempsey was a bad matchup for Marciano. Though Rocky does have the edge in one-punch power and possesses a slightly better chin, Dempsey has the advantage in handspeed and has a much more exspansive arsenal of punches. I think Dempsey would land two punches to every one Marciano lands. Add in the fact that Rocky was somewhat of a slow starter and Dempsey was one of the quickest and you have a tough day in the office for Rocky. I think Dempsey's initial onslaught would take a lot out of Marciano, leaving him too hurt to stage a comeback. Rocky definately has his moments, but Dempsey stops him on cuts late in the fight.