I look at the resume's of both Muhammad Ali and Evander Holyfield and it doesn't appear that they fought almost all of the serious contenders in their respective eras. I suppose you could say that Holyfield missed Ike Ibeabuchi, but that really isn't his fault. Did either of these two miss any outstanding contenders? Also, what other champions fought everyone worth fighting in their reign? All weights count, not just heavyweight.
Off the top of my head, Holy missed Prime Tyson (not Holy's fault), Ruddock, Tua and Golota. But there were lotsa good heavies in the 90's.