Since you ask I would be verry confident of Dempsey or focused Tyson beating Baer but not Frazier or Marciano. See my stylistic reasoning?
With Tyson I do absolutely and it makes sense that Frazier or Marciano being guys who heated up as fights went on would have the early disadvantage against a KO monster. Dempsey though, even being a fast starter, is smaller than Frazier and near the same size as Marciano but fought with little tactical precision and had no use of an adequate guard. Frazier fought with good tacticality and threw more precise punches, Marciano to a lesser extent but more so than Dempsey. So Dempsey is more likely to just sit and exchange blows with Baer. I think that Baer knocks Dempsey out, Firpo, a crude slugger like Baer with a lesser chin, put him down. But I think Tyson, Marciano and Frazier easily beat Baer, with Baer having a slight chance against Marciano.
I think where we disagree is on Dempsey. Dempsey in my opinion was a better all round offensive fighter than Tyson. He had a similar style but was much more adaptable. Dempsey is the best infighter the heavyweight division ever produced. He threw more short punches on film than any other heavyweight champion including Joe Louis. His precision was spot on. He basicaly stoped the No1 contender (Fred Fulton) with a single combination. A body shot wrecked him and then he finished the job. You are seriously not seing the whole picture with Dempsey.