I'm as big a Hagler fan as anybody, but he didn't have one punch explosive power like G-man and Jackson did. I don't know much about Stanley Ketchel, Bob Fitzsimmons or Sam Langford, so I won't rank them here. My top 3: 1. Gerald McLellan 2. Julian Jackson 3. Motor City Cobra
Hearns didn't have many knockouts at middleweight, but his knockout of James Shuler, and the way he stunned Hagler the way no other guy I ever saw stun him means something. John Mugabi was probably a hard puncher at middleweight, but he didn't have the fights there to prove it. Most of his good knockouts were at 154 and against lower level guys.
Let's talk middleweights. Hagler 78% KOs, Fitz 76 (was Fitz really a mw) , and Ketchell 75. Incredible ko percentages. But, let's narrow it down to the hardest punchers with at least 50 fights, and many of those as a top mw. I think that the mw with hardest right hand punch is Rocky Graziano 63% KOs, and the hardest left hand punch was Cuba's Florentino Fernandez 64% KOs. Both were knockout or be knocked out type fighters. Always dangerous despite their weak chins.