If you take on-screen literally, then it's Imu, of course, when he destroyed an entire country.
If you take on-screen "as written/mentioned in the manga in text form", then it will be (as already written above) - Lucci and Shiryu. Both are bloodthirsty sadists, Lucci also worked as a hired government assassin, and Shiryu had almost unlimited power in Impel Down.
Kaido, in my opinion, is not a sadist; judging by his dialogues, he simply destroys everything that stands in the way of his goals (yeah someone will die probably, but its not Kaido's "problem" or goal to kill as many). His personality doesnt get along with "the character who killed the most".