YOu dont have to be undefeated but I think there should be a win % . If you have 10 losses you probably cannot be an ATG, unless you have 140 wins. But if you are 57-10, there is NO WAY you can be an ATG. I think it goes first and foremost by losses. Marciano was an ATG. Foreman with only 4 losses in 70 fights---ATG. Joe Louis with only 2 losses---ATG. Ali only 5 losses----ATG. Frazier only 5 losses----ATG. Larry Holmes---ATG all the way---only a few losses---18 straight title defenses. No doubt about Larry. Mike Tyson----not sure. Not sure about Tyson.

An ATG is NOT a Buster Douglas who has 1 night of pure glory and then fades from all stardom. Leon Spinks is NOT, even though he beat the great Ali. Gene Tunney 's only claims to fame are his 2 wins over Dempsey. Tunney cannot be an ATG.

Number of losses, longevity, title defenses. That is it for me.