Not heavyweight, but Arguello-Mancini comes to mind. You could kinda/sorta discuss Holmes-Witherspoon and Holmes-Weaver, the only problem being no one really became the man after Holmes until Tyson came along. But Weaver and Witherspoon at least each had 15 minutes of fame in the heavyweight limelight.
Witherspoon was a massive underdog and this was the actual fight that had the experts inclined to think Holmes was on the way out. The true passing of the torch should have been Holmes - Tyson, but various occurrences killed it off.