I can absolutely see the Sibson contest being listed as his peak. I think I would vote the Scypion fight a few months later. Either way, Marvin was a monster at that point.
It'll be against a flat-footed, face-first brawler who came forward in straight lines and had no other approach beyond that. God, the threads in this section are so utterly generic.
I thought he fought great and it was one of his best performances, even though he was past his best. He showed many of the qualities that made him great and had a lot to fall back on after he had declined in certain areas. He still showed his brilliant jab throughout the fight. His legs and reflexes had slowed, so he was getting hit more. By a monster puncher. But he had the great punch resistance, will, and conditioning to fall back on despite taking so many hard body shots that he urinated blood. He couldn't use the ring like he usually did, and ended up spending sequences in the fight just going to war. Round 6 was a classic. By rounds 8 or 9 on, he spent a lot of it tearing Mugabi up with close shots on the inside. But it wasn't all on the inside, he still jabbed the hell out of Mugabi from outside. By the end, Hagler had beaten all the fight out of Mugabi.
On footage, probably Monroe III, Seales III and Hamani. In a demonstration of stick and move versatility, Bennie Briscoe is obviously his definitive performance as a cutie. So far as tracking down a runner with great legs himself, I'm not aware of Hagler-Mike Colbert existing on known footage. You do have to look at his pre-title resume for most variations from this.
