I’d say Gans’ fight with Nelson could count, though I’d also nominate Langford’s methodical destruction of Jeannette up there too!!
Probably not the earliest, but I've always loved watching Canzoneri take guys apart. Some of his fights with Ross, Chocolate, Berg and Ambers are the best pre-WW2 technical showings I've seen. It's almost like a period of time in limbo, stuck between the Walker Law and the more modern looking guys in the 40s. I'm sure there's better earlier footage, but I've not actually watched anything prior to 1919 in like 3 or 4 years.
for me too, Tony Canzoneri was the first time I reckoned with the fact that by the mid 1920's, most Boxers had become the Athletic Stylistic Athletes that we are used to seeing and expect from Top Class Fighters... sure there would have been earlier examples of the athlete types over Prizefighting 'grit & grind' teak tough scrappers. But by then it was the advent of 'common place' Skilled Athletic Craftsmen. both the Preservation & Proper Technic of actually 'using' recording equipment too, is a big factor, yes. But yeah, Tony C, for me.
For me, Johnson vs Jeffries possibly. Jeff well past his best of course but Johnson, perfectly conditioned and tuned, still made all the right moves and we get to see a broad selection of Johnson’s armoury on display. Johnson’s own meticulous prep reflected a good measure of respect toward Jeffries and proved that Jack wasn’t taking any chances in the event that Jeff did recapture some of his formidable, former self.