Impact. Tyson blowing away Spinks in 91 seconds became iconic, Tyson was awesome! Foreman vs Frazier 1 became iconic too, Cowsell: "Frazier goes down!" "Frazier goes down!" iconic, Foreman was a monster.
'Down goes Frazier' Mark, not 'Frazier goes down'. You've made it about as iconic as that famous saying 'Hitler heil'.
The definition of these things shifted over the years, particularly with the rise of TV and later instant video everywhere. These weren’t originally conceived as we see them now, near as can be told — not the ‘best’ fight or round but the one that defined the year, would be remembered and made the biggest impact/impression. When they tell the tale of the year, people will remember George absolutely destroying a Frazier who was perceived as an irresistible force, a tank who came forward and willed his way until the opponent wilted. Foreman shattered that perception and signaled a sea change in how the division was seen … a new (and frightening) sheriff was in town, so to speak. Likewise, Tyson’s 91 seconds vs Spinks cemented him as the absolute ‘baddest man on the planet,’ which wasn’t settled until he won the lineal title. He had to beat the man (Spinks) who beat the man (Holmes) to plant his flag in firm ground. Over time, these awards came to be more ‘what’s the best/most competitive/thrilling’ round/fight of the year. Until a certain point (which isn’t a line in the sand, more of a transition over time), it was ‘what’s going to be remembered as the most historic’ fight/round of the year.