I think there are many fighters who can end a fight with a single punch. As for names, well Kovalev, GGG, Thurman and Nicholas Walters are four very big punchers that stick out for me at the moment, just my personal opinion.
Wlad Stevenson Kovalev Golovkin Mathysse Concepcion Gilberto Ramirez Stevens Lemieux Uchiyama Mikey Garcia Keith Thurman Takashi Miura Johny Gonzalez
The obvious ones are Stevenson and Kovalev. If we wanna stretch it, we can add Matthysse, Golovkin, Thurman, Lemieux and Wlad.