Jack Dempsey vs. Luis Angel Firpo (MUST WATCH FIGHT)
Honestly, I think Dempsey was overrated as a fighter. His big wins came against either large fighters with little boxing skill, or much smaller competitors (Georges Carpentier was like taking on Oscar De La Hoya). The advantage Dempsey had was that he could knock down a fighter, swarm over him and punch him immediately when he rose, before the downed fighter could even reposition himself. Check out footage of the Jess Willard fight for an example of that. Gene Tunney was the most skilled boxer Dempsey fought, and Tunney dominated the action in 19 of 20 rounds.
Not to discredit Dempsey. Boxing was just fought differently in those times. I'm not certain a fighter could come from that era and win today without adjusting his style.