I consider Usyk's feat to be the greater one. Foreman despite his ferocious power was easier to outbox.
Foreman had the reputation of being invincible. He has shown no weakness to this point. In terms of expectations, the Rumble in the Jungle was the bigger story. Ali has ice cold exploited 2 flaws that people were not aware they existed. Fury had already shown some weaker performances and people were aware that he was vulnerable. But in terms of athletic performance, Usyk was sensational. As a natural cruiserweight, Usyk has beaten a far bigger, younger, undefeated opponent in top condition, who has no real deficits in skillset and athleticism. I think that Usyk also boxed against the whole boxing industry including the ref to a certain extent. some people make the point that the ref saved Fury from TKO. But if you think about it, the financial interest in Fury winning or drawing the fight was huge. Fury is the better known, bigger, louder, more marketable man. Fury vs AJ alone might be enough reason. + I think bigger boxers are allowed to fight dirtier in this era. Ali might made it acceptable to use hight and range a little bit in a dirty way, but Lennox, WK and Fury took it on a way other Level. So i am full of respect for that performance of Usyk!