In the Star Wars universe, the Sith represent the greatest threat to the Jedi and the galaxy at large. As practitioners of the Dark Side, the Sith are adept at wreaking chaos and destruction. There have been many Sith Lords throughout Star Wars history, but only a handful of them received major screentime in the movies and TV shows.
In fact, some of the lore's most interesting Sith appeared in other mediums, such as comic books, novels, and video games. Longtime fans of Star Wars are likely to recognize some of these evil individuals. Whether or not these Sith Lords get a live-action adaptation in the future is anyone's guess.
6 Darth Traya
Darth Traya, a former Jedi Master named Kreia, is arguably one of the Star Wars universe's most interesting and enigmatic villains. Kreia was exiled from the Jedi Order after several of her students, including Revan, fell to the Dark Side. Kreia took the mantle of a Sith Lord herself and became Darth Traya.
She formed a triumvirate of Sith alongside two of her students, Darth Sion and Darth Nihilus. However, Nihilus and Sion overthrew their master and cast Kreia out of the Sith Order. During the events of Star Wars: Knights of the Old Republic 2, Kreia seeks to destroy both the Jedi and Sith for betraying her by manipulating the protagonist. It's certainly one of the more compelling character arcs in a Star Wars game.
5 Darth Vitiate – The Sith Emperor
Darth Sidious is often seen as the most powerful Sith Lord in history, judging by what he manages to accomplish. Nevertheless, the Sith Emperor from Star Wars: The Old Republic, can definitely give Palpatine a run for his money.
The Sith Emperor, once known as Darth Vitiate, was exceedingly powerful in the Dark Side. He consumed all the inhabitants of his home planet in a bizarre Sith ritual that essentially made him immortal. The Emperor rebuilt the remains of the Sith Empire and nearly conquered the entire galaxy in a war with the Republic.
Even more remarkably, Darth Vitiate built a separate power base centered on the planet Zakuul, known as the Eternal Empire (where he ruled as Valkorion). Darth Vitiate also managed to cheat death on several occasions, though with a bit more success when compared to Palpatine.
4 Darth Plagueis
Speaking of Emperor Palpatine, it's worth mentioning his enigmatic master, Darth Plagueis. He's a character that is first mentioned in Revenge of the Sith, in a story that Palpatine recounts to Anakin. It wasn't until James Luceno's aptly titled Darth Plagueis novel that the character was fully fleshed out.
Darth Plagueis, like many of his peers, was obsessed with the concept of eternal life. In fact, it's heavily implied that Darth Plagueis' experiments with midi-chlorians resulted in the birth of Anakin Skywalker, a boy conceived without a father. Plagueis also engineered many of the events leading up to, and including, The Phantom Menace.
3 Darth Caedus
In the Legends continuity (previously the Expanded Universe), the Skywalker-Solo extended family experiences quite a few personal tragedies. Arguably the saddest of these troubles revolve around Han and Leia's three children: Jacen, Jaina, and Anakin.
Under the tutelage of Luke Skywalker, all three children became Jedi Knights by the time they grew into adulthood. Jacen Solo, however, eventually fell to the Dark Side out of a desire to protect the galaxy by any means necessary. He took the name Darth Caedus for his new identity. Tragically, his own sister Jaina was forced to kill Caedus in order to put an end to his tyranny.
2 Exar Kun
As stated previously, the lore of the Star Wars universe is filled with Sith Lords, many of which are from the galaxy's early history. One of the most influential Sith Lords from the Old Republic era was Exar Kun, a former Jedi who was seduced by forbidden Sith knowledge.
Exar Kun slew his Jedi Master in cold blood and led a rebellion against the Republic in an event known as the Great Sith War. Kun nearly succeeded in his endeavors but was eventually defeated. He fled to Yavin 4, where he managed to preserve his spirit inside a temple. He was killed permanently by Luke Skywalker and his students several years after the Battle of Endor.
1 Naga Sadow
One of the galaxy's most traumatic conflicts was the Great Hyperspace War fought between the Jedi/Republic and Sith Empire. The war was perpetrated by Naga Sadow, a Dark Lord of the Sith who believed that an invasion of the Republic was possible.
Although Sadow nearly won the war, he was betrayed from within and defeated. Much like Exar Kun would do hundreds of years later, Sadow fled to Yavin 4, where he constructed several temples (like the ones seen in the movies). He placed himself in suspended animation for over 1,000 years before he was killed by a fallen Jedi curious about Sith teachings.