I wonder if the one-punch knockout is what seduced most boxing fans at the outset and they learned to appreciate the rest.
The one-punch knockout has immense drama closing out a fight before it's started (Tyson's mystique was built on this). Sometimes, as with Gatti's career and George Foreman's stunning knockout of Michael Moorer, the punch arrives to save a fighter from a sure loss and turns everything around. Other times a fighter claims a fight courtesy of a split-second punch at the last second. There are any number of other variations.
The one-punch knockout is the great equalizer. Even when the fight isn't decided by that lethal punch, it can still be the abiding image all the same (Ali vs. Frazier I).
There's quite a spectrum of these blows to choose from, but we've got a pretty fair cross section of eras included. I haven't placed them in any special order, however the greatest fighter who ever lived has his offering saved for last.