Who was Usyk's best non-Briedis cruiserweight opponent?

Hope you don't mind me taking Briedis out. Many observers still consider him to be Usyk's toughest fight, period, and his record of wins over Huck, Glowacki, Dorticos, Charr, Perez, Durodola,...

Who was Usyk's best non-Briedis cruiserweight opponent?