I'm tempted to pick 1996 Holyfield. Knowing our fighters here, both Holyfields are going to get drawn into a trading match. 1996 Holyfield is the bigger, harder hitting man.
He'd become a superhero brought to life by 96, enough gear in him to win Mr Olympia. It just feels like the 91 version would be bullied and beaten up by the guy that fought Tyson.
Older Holyfield had on and off health problems and reduced work rate. At the same time, the older version also knew all of his own tricks from the younger years. Might be interesting
1996 Holyfield. He was more juiced up, thus bigger and stronger, and with a better chin. He was also more experienced and battle hardened. This is a great thread btw.
It's tough to say, but I think '91 Holyfield. Less savvy, but significantly more active. Of course, this is ignoring the psychological effect of fighting your past/future self, but there's too much to get into regarding that.