Easy one here. Ezzard Charles was famous for his endurance. He was the first boxer that could go the whole 15 rounds against Rocky Marciano, losing by points. Now when an out-of-prime Charles could go 15 rounds against Rocky Marciano, without getting knocked out and Marciano hit harder than Dempsey(Dempsey himself said so in an interview and Marciano for a while was trained by Dempsey, so he could see the diffrence) and hit Charles much more often than Dempsey would(since Rocky's stamina level was from another world, Jacks was just medium), he would definitely go the whole 15 rounds against Dempsey without getting knocked out. Now since Marciano never ran out of gas, but Dempsey would, Charles can win the later rounds by points and take the decision over Dempsey.

Charles by 15 UD

Ezzard Charles in his prime best was one of the most compleat fighter in the history of the sport. Even Ali admitted to being influenced by and copying some of Ezzard's style of fighting. Prime for prime I think Charles who fought already in the modern era of boxing handily beats Dempsey on points.

Ezzred Charles was a truly amazing fighter, But Dempsey in his prime would Mollywop Charles without much effort, prime dempsey was just in a league of his own

Dempsey win KO

ezzard gives dempsey a lot of trouble, but he manassa mauler does enough to take a ud.