If cruiser yes, then the answer is yes. The question did not mention a weight calls, and winning heavy after being and ATG cruiser would only increase his status.
Usyk has the best CW resume ever by far, aka CW GOAT. If you're the GOAT of any division with 4 titles you're ATG
Nobody can answer that stupid question of an active unbeaten fighter today, regardless of the weight class, compare when they have finished their careers, only then can we look back and see how one would have faired against another fighter from a different era, but if i had to pick a fighter who would beat Usyk at this stage in his career, i would say Holyfield would stop him late, his aggression and sheer will to win would see him through, but like i say lets see at the end of his career, i suspect he will have a couple of Ls on it, but will more than likely become a great.
Usyk fought in the weakest Cruiserweight and Heavyweight era ever. No fault of his but it's a fact. Hall of fame but not ATG.
No because i think that term should be reserved for the absolute very best in the history of the sport who achieve the nearly impossible, I'm talking the top 20 to 30 fighters ever. Holyfield has already done everything Usyk has done against better fighters. I think lumping Usyk in the same category as the Sugar Rays and Henry Armstrongs feels wrong, if Usyk is an ATG, is their a better term to describe them? I'd probably describe Usyk as a modern great, not all time
What's all this cobblers about 'CW yes, HW no' FFS? He's arguably the best CW there's ever been: unifued, undisputed CW champion, beating the best in the division, away from home and usually in convincing fashion and then takes 3 belts off a decent HW champion in convincing fashion again, in the other guys back yard. If Fury has retired, who beats Usyk at HW right now? Of course he's an ATG.
Holyfield's greatest feat is going 1-2 vs Bowe. "Better opposition", no way. He lost vs Lennox, that's great opposition, but he got battered twice. His CW career totals beating a a mediocre champion, one of the worst champions ever, and a 5'5 LHW (who some thought beat him). 42 year old Foreman, 42 year old Holmes, Buster Douglas and Ray Mercer must be a lot better than I remember. The only elite/close to elite opponents he beat (elite when he fought them) both beat him too, and many (if not the majority) feel that he should be 0-3 vs Bowe.
Hollyfield is at present the CW that has had the most success at HW. Although this can be surpassed by Usyk. Usyk at CW though, has far superior record compared to Hollyfield (who fought in a very weak CW divison at the time).