These are my 2 favorite knockout artists. Tough to pick what my favorites are. For Tyson, I guess I'd go with the Pinklon Thomas one. As for Louis, this might be even tougher to pick.
So many of early Tyson's KO's were impressive. When he was disciplined in the ring, kept his hands up, bobbed and weaved a bit (more Patterson than Frazier), used his footspeed, everything. One particular Joe Louis KO impresses me, tho' I've suddenly forgotten the opponent's name. Anyway, the guy had a pesky guard that was giving Louis a little trouble. Joe uses a left hook to knock the fellow's left hand away, then nails his now-open noggin' with a perfect, short right hand. Beautiful thing. Very rarely do you see a fighter going after hands, arms and shoulders to get at an opponent. Foreman would pound on shoulders and arms pretty effectively (didn't work against Ali, tho'). Frazier would occasionally pull a guy's arms or hands down but it was more a frustration move than anything effective.
I like the way Louis knocks Carnera down while walking forward. An emotionless killing machine that throws punches while fluently walking forward, right there. The one where he steps back out of range of a right hand, then throws a hard right uppercut followed by a left hook right on the chin is a great one too. I keep forgetting the opponents name, was that Paycheck? Of Tyson there are also several. One that i like is how he finished Bruno on the ropes with a right uppercut / left hook and several punches before that. It wasn't a clean KO but he looked so ferocious, the same "killing machine" impression that i got from Louis in the Carnera one.
What made them both so fun to watch is that every KO was so beautiful and the stuff that makes hili reels. You could just string together their kos and sit back in awe. I think the two coolest looking Tyson KOs were the Lorenzo Boyd double right, and the slight pivot/spin move he did just before sneaking in the sneakiest and shortest of right uppercuts on Mark Young. But there are so many. Joe Louis' demolishions of Max Baer, Buddy Baer II, Abe Simon II, the Conn I KO sequence. Basically any KO of Louis or Tyson are so fun to watch. I guess that's why fans enjoy watching them so much.
From Tyson I would say Pinklon Thomas. From Louis I would say Harry Thomas. A left hook staggered Thomas against the ropes and Louis pulled his arm back like he was hesitating and then quickly fired another left hook at the helpless Harry and layed him out. Brutal, but niiiiiiiiice!:good Funny that both ko's I mentioned were performed on guys both named Thomas.
As far as I know, the Harry THomas KO was not filmed. Do you have that on film or something, or could it be another fight?