In terms of being the best ever, Fury is certainly the best of his era. I think he beats AJ and whilst Usyk is every bit as good a boxer, he'd just be too small. The only slight question is that he hasn't actually fought either of them, due to Covid and all sorts of other reasons. In that sense there isn't the 1970s thing where the best all fought each other (Ali/Frasier/Norton/Foreman). In fact, Fury's list of opponents since Klitschko is Siferi, Pianata, Schwartz, Wilder, Wilder, Wilder, Wallin and Whtye. And it wasn't that stellar before Klitschko.
Probably the moment to celebrate his career rather than question his CV and I do think he's the best of recent years in that he has quite different ways of winning fights. He's shown he's the best of his era, though with a limited number of elite opponents, iyswim.