Round one of Patterson-Johansson III. Any punch might produce a knockdown, and it could come from either man. I don't know that I've ever seen a knockdown produced by a fighter in more serious trouble than Floyd was at the end of that opening round. Ingo had knocked him completely silly.
For the stakes, drama, and definitive changing of the tide toward the challenger, Round 9 of Tyson/Douglas is an epic.
Jack Dempsey vs Jess Willard. If the scoring was on the 10-Point Must system. Dempsey would have had a 10-2 Round, 1st Round. And Jess Willard was still around for Rounds 2 and 3.
What's the most lopsided score for a single completed round on the ten point must system you're aware of? (To give folks an idea of how recent an advent the ten point must system is, Ali didn't compete in one until 1978, when he was dethroned in the ring for the only time, so there's not much more than three decades of substantial history here. For a brief shining moment, both the championship distance and ten point must system co-existed.) Due to a calculation error, I know that Bob Martin scored round four of Pintor-Zarate 10-6 for Carlos, making his 145-133 card in favor of Zarate more lopsided than he intended.
One of the Marquez-Pacquiao I judges had a 10-6 round. It's rare, rare because of the three KD rule of course.
Hasn't been said yet and may not equal the excellent prior posts, but Delahoya-Quartey round 6 was solid with both men hitting the canvas.
http://www.dailymotion.com/video/x8n4ah_guebbas-vs-hunt_sport 7:00 2nd round guebbas sees the blood and a rare win on the cards, danny hunt seeing his career in the pro ranks looks to bring the heat and back him up.