Probably Ali/Liston, because at the time (from what I understand) Ali was looked at as a lot of talk, people didn't like him so if he lost to Liston it would have taken him a looong time to work his way back up. He probably wouldn't have become a huge name before the war, and assuming he still dodged the draft, probably would have never become a big name.

The other one that comes to mind is Louis/Schmelling 2. I guess if Max whipped Joe again, it would have been a huge blow to boxing and morale in the US.