How may men who were never #1 overall ( Lineal or Ring magazine champion ) in their day would likely #1 today? Assume their prime started in 2015, and they get a title shot. Alexander Povetkin. Gave Joshua a tough fight at the age of 39, breaking his nose and rocking him. If he was in his prime for this fight, he likely wins it. Povektin matched up just fine vs bigger guys in Price and Wach, and IMO would KO Wilder. Fury would be a tough fight, I don't think Fury could deal with his infighting skills. Tim Witherspoon. In what could be viewed as an early version of a super heavyweight with skills, Witherspooon had an all-around game. good jab, a powerful right hand, good defensive and countering skills, could take a punch. I'll list a few more names, from not that far back: IKe Ibeabuchi - Very likely Sultan Ibragimov - Maybe Ruslan Chagaev - Not likely Corrie Sanders - Maybe Kenny Noton - Maybe Ray Mercer And what would a thread be without Tua? List whomever you think.
I agree with most of your names except Mercer. Mercer was excellent but he had a tendency to underachieve at times. Povetkin, Witherspoon, - YES Ibeaubuchi and Sanders- likely Tua and Norton- maybe Bruno- maybe. I could see Bruno beating Joshua (50-50) but losing to Fury. Was Bruno ever #1? Morrison and Ruddock- unlikely, and i am a fan of both. I could see them winning a belt but not being #1 Shavers- NO. He could cause upsets but he isn't durable enough. No way. Not too many others in recent memory.
If we go back in time a little bit, Buddy Baer would be a decent pick. I don't think he was as skilled as Fury or Joshua but he was huge and had a big punch. He could cause some upsets but would be an underdog vs Fury and Joshua. So Baer would be a no for me, but like Shavers, he could cause a few upsets.