Golovkin doesn't have tremendous 1 punch power. That's never been his strength or his style. His punches are thudding and accurate, which break down opponents.
GGG has rubio, geale, macklin, simon, and ishida. All 5 except for rubio have never been koed by one shot prior to facing ggg.
GGG has more one punch KO's than that but others who also would have would include Deontay Wilder, Adonis Stevenson, Sergey Kovalev, Wladimir Klitschko....other than that I'd guess maybe Jermall Charlo and Mikey Garcia. There are probably a few more that I haven't listed.