Fighting in general, Ali-Frazier 1.
Like my name says, I'm an old guy, though I was still a kid then. But the buildup to that was insane, it was on national TV every day (and back before cable when there were only a few channels), on every newspaper, everyone was talking about it for a month before the fight - it was the biggest sporting event of the year by a long shot, bigger than Superbowl or World Series or Stanley Cup. Some of that was no doubt related to Ali's stand on the Vietnam War, it became more than about the heavyweight championship. Sadly enough it was close circuit so I only saw it in full years later.
In MMA, Fedor-Crocop.