1. Marka Ragnos

A number of noteworthy Lords have ruled various Sith empires since the Star Wars timeline began, but Marka Ragnos was the only one to create a so-called Golden Age for his Sith Empire. He used his intelligence rather than physical strength to protect his position, and ruled undisputed for a century.

But that doesn't mean that he wasn't a powerful user of the Force. As a matter of fact, he was so skilled in using the Dark Side of the Force that he appeared as his own ghost at his own funeral (a trick that we also saw much later in the movie timeline). As another neat trick, he was also able to preserve the essence of his body for 5000 years after he died.

Marka Ragnos was unique among users of the Force because he employed a special sword of the primitive Sith race that was imbued with Sith sorcery. He used his Scepter to store Dark energy and release it toward others, which made them Force-sensitive. A pair of gauntlets (later used by another powerful Sith) further enhanced his abilities and sword-fighting skills.

The people who actually made Star Wars described Marka Ragnos as the most powerful Sith of all time. We'd love to see some version of him in future movies.