Among those you’ll find nearly impossible to beat in fantasy matchups on this forum (aka darlings) who didn’t accomplish all that in their careers are: Corrie Sanders David Tua John Conteh Jake LaMotta (more accomplished but also had more than his share of losses that people brush off)
If we count reddit as a forum then for sure it's GGG.. There's some folks on r/boxing who would genuinely bet all their live savings on GGG beating Galactus.
Sanders compares favorably to the forum version of Witherspoon who was somehow both in shape and experienced at the same time.
70s Foreman is the king by far in his prime he was a small primitive HW and overall his best three wins are… Moorer (glass) Norton (glass) and a shot, small alcoholic in Frazier who prepared for him the way he did for Stander and Daniel’s. THAT GUY destroys Holmes, Louis, Ali in a rematch, Holyfield, Wlad, Vitali, Lennox etc he’s like a big Marciano (famously without the stamina) with how his fan base acts like he overcomes every physical disadvantage and lack of sophistication with the magical ability to “find a way”