I actually think that a lot more punchers could have stopped Chuvalo than most think. The only two world class hitters he ever faced were a 16 fight Frazier and a 20 fight Foreman - both of whom stopped him early ( though one was a controversial stoppage. ) With all this being said, I'll go with Dempsey to break him down sometime mid rounds. This isn't lasting 20..
Chuvalos main problem was he was so slow. Big fast punchers would turn him into hamburger as he just was too slow to defend himself. Dempsey within five rounds in a horrible mismatch.