Darth Krayt and Darth Sidious. Sidious was the most powerful Sith, but Krayt was the last, so far (there may be more coming in future Legends novels). And Sidious, like many Sith, made Hitler seem like nothing, in terms of people killed:
Hitler and the Nazis: 3-4 million people over a period of 10 years
Sidious, Vader, and the 2 Death Stars: up to 1 trillion people over a period of 23 years, including those on Alderaan (or any other Death Star Target)
That's anywhere from 108,695 to 144,927 times the mortality rate, per year! Yikes! I wouldn't want them ruling ANY galaxy, not even IC 1101!