I mean I don't know enough about some the oldest era's so maybe idksab but Wlad's achievements and how long he lasted for are kind of insane and I think it's really hard to top. I would imagine the majority would say Rocky Marciano.
Usyk, Klitschko x2, Fury, Dempsey, Corbett, Tunney, Fitz, John L and Marciano - Tunney don’t really do all that much however.
A completely washed up Dempsey, whose wang was sore from 4 years of starlet abrasion, over ten rain soaked rounds where Jack couldn't set for a punch, and then again was declared winner after old, stale Jack KO'd him. And lightheavy Gibbons (he was in no way a HOF heavyweight) in his final fight of a l o n g career. Yeah, he's WAY up there.
The best LHWs of that era would clap Usyk like they were clapping their hands. Gibbons has the longest unbeaten record of any HW or LHW notable. Longer than Rocky Marciano. Of course he deserves to be in the HOF regardless of what you think about him.