Fight reports attest to the damage he did to Sharkey, Jeffries, Choynski... and Ruhlin, who he damn near killed.
Marciano ruined a few guys,Vingo, Layne,Lastarza,Charles,C*ckell. Louis smashed Uzcudun's gold teeth, knocked them on to the ring canvas drove Braddock's teeth through his mouthpiece and into his lips, broke a bone in Schmeling's back,turned Godoys face into hamburger.Bob Fitzsimmons nearly killed Gus Ruhlin, so badly was Ruhlin punished he had to sleep on a make shift cot in MadisonSquare Garden and then spend a week at his managers convalescing and two men died after sharing a ring with him.Fitz broke Jeffries nose, and caused deep lacerations to his cheek bones and eyebrows. Johnson gave Burns an injury to his jaw that made it swell up alarmingly,he punished McVey so badly in their third fight McVey was looking like a gargoyle at the end. Duran gave out some bad beatings , notably to Davey Moore.
Depends if you mean ko power or the fact that opponents were changed by the experience. If the latter, I'd say Chavez.
Jose Luis Castillo put some bad beatings on opponents over the years. Not surprising considering he was basically a poor mans Julio Cesar Chavez, who was a master at being a damaging fighter. I remember him doing it against a prospect back in 2011 in a televised bout. Guys name was Ivan Popoca. Even though Castillo was years past his best, he was still a meat grinder of a fighter if you weren't prepared or quite good enough. Popoca never fought again.
Joe Louis. “It ain’t like a punch. It’s like someone nailed you with a crowbar. I thought half my head was blowed off. I figured he caved it in. After he hit me I couldn’t even feel if it was there.”—James J. Braddock “God how that man can hit. I can’t remember anything after the first knockdown.”—Johnny Paycheck. “He hit me eighteen times as I was in the act of falling.”—Max Baer “I was worried he would break someone’s neck.”—Arthur Donovan Ray Robinson. “I don’t know anything about that punch. Except I watched it on movies a couple of times.”—Gene Fullmer “He was the best puncher, he was the hardest puncher.” - Carmen Basilio Bob Fitzsimmons. “Whilst physicians were examining him, Ruhlin opened his eyes and faintly asked for water. This was given him as he again lapsed into a sort of stupor. Blood at this time was trickling from his ears and nose.” Sam Langford. “I fought most of the heavyweights including Dempsey and Johnson, but Sam could stretch a guy colder than any of them. When Langford hit me it felt like someone slugged me with a baseball bat. It was like taking ether. You just went to sleep…”—“Fireman” Jim Flynn Sonny Liston. “After Liston knocked me out I felt, for four or five seconds, that everybody in the arena was in the ring with me, circled around me, like family…I actually blew a kiss to the crowd from the ring.”—Floyd Patterson “Nobody should be hit like that. I think about it now I hurt. He hit me to the stomach with a left hand in the sixth. That wasn’t a knockdown. It couldn’t be. I was paralyzed. I couldn’t move. I couldn’t move enough to fall down.” - Marty Marshall If I was naming one guy, might name Langford. Ketchel was a beast too.
Jeff Fenech Wilfredo Gomez Gerald McClellan Nigel Benn Tito Trinidad Terry Norris Recently, George Groves and Gennady Golovkin.
Someone like Frank Bruno who hit really hard but lacked the skill to finish opponents so they had to keep taking the heavy stuff