Ike Williams might be the best example at 39.6 percent (upon Ctrl+F'ing the pages of this thread, I see I was beaten to that one, along with Ron Lyle). How 'bout the guy who is most critics' pick for GOAT? 56.6 percent. Graziano at 62.6 percent. Bummy Davis at 58.2 percent. Much more deeply competitive and active times, though. Relatively more recent, Matt Franklin could certainly crack, but only 50 percent.
At HW Travis Walker could really crack. He hurt or dropped almost everyone he fought, but like a low rent Earnie Shavers he could never finish the job. Off the top of my head he hurt/dropped: Pulev, Chagaev, Arreola, Browne, Adamek, Leapai, and even Vitali (in sparring). Yet his knockout ratio was dire because, frankly, he was a bit crap.
You know, all factors considered, Darnell Boone should really get a mention here (24 percent). Not really a "big puncher", though, Jake. Spoken as a massive fan.
Derrick Gainer could punch with a sub 50% KO Ratio. Fernando Montiel was a good puncher with just above a 60% KO Ratio. Luiz Callazo with a 42% KO Ratio. Azumah Nelson has a 58% KO Ratio but could crack.