I rate Louis the best finisher in history. Based on available footage, I put Tyson over Dempsey. But Dempsey is at a huge handicap here as we can't see most of his knockouts.
They are almost interchangeable, and it is hard to divide them. I will say that Dempsey was significantly smaller than the other two, and that might give him the argument in a pound for pound sense.
I can't think of any Louis opponents who were hurt by Louis and still beat him, or fighters who Louis hurt who stayed in there much longer, if he had you hurt he was going to finish you, he was a ruthless finisher because he didn't panic or get over anxious when he hurt you he just methodically dispatched the oponnent with hard, pinpoint, devastating combinations of punches. Tyson was as good a puncher as Louis but he was more inclined to frustration and getting over-anxious or over excited when he hurt his opponent and didn't always finish them off, look at what he did to Jesse Ferguson but he couldn't finish him because he didn't go back to what got him there he just wailed away with telegraphed punches, he improved a lot after this fight but was always prone to these emotions.
Dempsey was the smallest of the three, but he destroyed bigger men quicker than the other two! The trade-off with that is that he probably had to take greater risks to do it. He looks invincible against Fulton and Willard, but Firpo took him to the edge of disaster. Perhaps Louis and Tyson gave you less of a punchers chance?
Louis would destroy an old, untrained, rusty Willard... as would most decent champs. The Fulton fight kinda wreaks if you believe Fred.