Any other era? I'm fairly certain Usyk beats Corbett, Braddock, Schmeling, Carnera. Would be very competitive versus Johnson, Dempsey, Louis, Charles, Walcott, Marciano.
Not too sure about Holmes, Frazier, Holyfield. And he's not beating Ali.
The massively overrated Ali would have virtually no chance against Usyk if we're objective, which 80-90% of boxing fans are not.
1966 Ali vs 2018 Usyk
Ali: 24 years old, started boxing at age 12, 88 confirmed amateur bouts with a record of 80-8 (3 rounders, 247 rounds max), 25 pro (144 rounds contested), stopped 9 pro opponents who were 197+ lbs
Usyk: 31 years old, started boxing at age 15, 350 amateur bouts with a record of 335-15 (3-4 rounders, 1076-1332 rounds max), 6 WSB (25 rounds contested), 14 pro (105 rounds contested), stopped 11 WSB/pro opponents who were 197+ lbs (pre-rehydration)
Ali had been knocked down from head shots twice in the previous 15 fights/4.5 years (Sonny Banks, Henry Cooper), was almost exclusively a head hunter, hadn’t fought a southpaw in 6 years, had lost to at least two southpaws in the amateurs (Kent Green by TKO2, Amos Johnson by SD3), 2013 Usyk was heavier than 28/33 of Ali’s pre-Mathis opponents (Ali himself was very big for his era) and as much as 9 lbs heavier than 1966 Ali, who had gone the distance with 5 of his 25 pro opponents (17-9-1 186 lbs Hunsaker, 15-11-1 225 lbs Sabedong, 18-7 189 lbs Johnson, 23-3-1 188 lbs Jones, 34-11-2 216 lbs Chuvalo), whereas Usyk (the best southpaw heavyweight of all time) hasn’t been stopped or dropped with a headshot in a combined total of 377 fights, amateur and pro and has an IQ which is almost certainly at least two standard deviations above Ali’s (78).
1966 Ali had a highly competitive fight with 49-2-3 European champion southpaw Mildenberger over 11.5 rounds (Ali admitted that he found Mildenberger’s stance and boxing ability very difficult to deal with), who was billed as being 6’1.5 with a 73 inch reach and 195 lbs (4-3-1 in non-KD rounds according to two of the three judges, 154-144 punches landed out of 612-538 thrown according to Compubox; by contrast Usyk outlanded the 6’2, 79.5 inch reach, 198 lbs, undefeated American Olympian Hunter 321-190, throwing 905-794 over 12 rounds). Mildenberger had been KO’d twice (once in the 1st round by 30-12-2, 201 lbs Dick Richardson, who had 2 stoppage wins in his previous 9 fights) and dropped numerous times in 54 pro fights, registered a 31% KO ratio with 0 KO’s in his previous 5 fights and had a 52-12 amateur record, with winning the German LHW championship being his best amateur accomplishment.
A more skilled, mature, durable and experienced 1973 Ali lost by a wide margin (117-112 Boxrec consensus, 233-171 punches landed according to Compubox) to Ken Norton: a suspect-chinned boxer with a padded 29-1 record and only 8 years of boxing experience since his amateur debut, who took up boxing in his early 20’s and had never been in a 12 rounder prior to facing Ali.
Usyk has studied Ali extensively, sparred and beaten many opponents influenced by him and has modern advantages in terms of training and "nutrition", as well as a tougher upbringing being from a working-class family in post-Soviet Ukraine, with a modern Eastern European style that Ali never experienced (the old Soviet school was limited to the amateurs in those days).
Ali’s objective chance = sub-5%.