I think the Holyfield fight, although a losing effort, was his last great fight. He took on an all-time great in their prime, and looked competitive. This George Foreman, to me, was still a "real deal". The last version of him that I'd seriously entertain in these sorts of conversations.
The rest of what he did during his comeback, doesn't really compare. You could see the physical decline even from the Holyfield fight, to the Moorer fight. He looked much more flabby and just unbalanced/uncoordinated. He was an old man who had been in his fair share of wars. The Holyfield and Stewart fights were pretty brutal, especially for a 40+ year old guy.
Despite the jokes about weight and hamburgers, I think the guy actually looked pretty impressive physically around that time (1991):
Lot of heavyweights out there now who'd wish to look as good as he did at that time.
By the Moorer fight you could see that he was starting to have an old man's physique: