Top fighters who have never been KD

Add Ruslan Chagaev, Alexander Povetkin and Nicolai Valuev to those. Ruiz landed two great right hands, Valuev backed up, then Ruiz followed up and didn't hit anything, and the big Russian tripped over his own feet... unless you stumble and are clearly out on your feet, you don't get knocked down three seconds after the punch lands....

Wrong, he was down twice against Bonavena, on short left hooks. It used to be on YouTube. Neither of them were officially ruled knockdowns (though they were), but Mayweather is excluded for a non-ruled KD too.