One of my favorites- Darth Tormen. Responsible for operations ending in the deaths of Supreme Chancellor Janarus, and Jedi battlemaster Jun Seros. Also responsible for having the government of Coreillia forced to secede from the Galactic Republic and join the Empire.
Years after Palpatine Krayt took the throne and shifted the balance of power in the galaxy back to the sith and led his own empire after defeating the Galactic Alliance.
Darth Caedus, formerly Jacen Solo, son of Han and Leia.
Darth Bane, founder of the Rule of Two and the Order of Sith Lords that eventually led to the rise of Darth Sidious
Darth Malgus- led the attack on the Jedi Temple during the Sacking of Coruscant, later betrayed the empire, declared himself emperor and was defeated on the planet of Ilum.
Darth Decimus, conqueror of Coreillia.
Darth Traya, lord of betrayal
Darth Nihilus, lord of hunger.
Darth Revan and his apprentice, Darth Malak.
Darth Andeddu, the earliest recorded Sith Lord to use the title Darth.