1. The Sith Emperor/Lord vitiate
Why: He reconstructed the sith empire and ruled it for over 1000 years making him one if not the longest ruling sith of all time.The Emperor made Revan and malak obey to his commands later starting the jedi civl war and discovery of the star forge. He was eventualy defeated by the hero of tython.
The Hero of Tython defeated the Voice,not the Emperor.
Abilities: Even as a child the emperor was very powerful with the force he killed his father with just a thought.He used the force to drain the life and force from a plane to achieve immortatiltyt. The emperor could easily dominate the minds of others to obey him. He easily defeated revan with his mastery of telekinesis.
Why: Ragnos clamined the title dark lord of the sith and became the emperor he took vitiate as an apprentice and taught him further in the ways of the sith. Whule he was in power it was known as the golden age of the sith.
It's unknown if Ragnos took Vitiate as an actual apprentice or just taught him a few things.
Abilities:Marka Ragnos was considered to be one of the most powerful sith of all time even after death he managed to keep his sprit alive and kept growing power he was nearly risen again on korriban but the jedi jaden korr defeated him.
6. Darth Revan
Why: Revan was the most powerful jedi of his time it was his leadership that allowed the republic to defeat the mandalorians in battle. He later became a sith sparking the jedi civil war on the galaxy he went to the star forge single handly and defeated his old apprentice darth malak and saving bastila shan. After the star forge he went into the unkowen regions back to defeat the true sith threat that only he knew about .
Debatable on whether Revan was the most powerful Jedi of his time, I'd say at the very least the Exile was far more powerful and had a larger,farther reaching impact on the galaxy as a whole.
Abilities: Revan was both strong with the froce and a lightsaber. He was one of the best tactians in all of the jedi. In revans time there was hardly any match for him besides the sith emperor himself who traped him in statis at the foundry.
9. Darth Plageuis
Why: He had the ability to create life he used this ability to create anakin skywalker inside of shimi. He taught darth sidious the ways of the force and the dark side.
The Force "conceived" Anakin in Shmi's womb in response to Plageuis and Sidious attempting to swing the balance of power towards the Dark Side.
Abilities:He was the master of the unnatrual aspects of the force He is best known for the ability to create life. Plagueis used 2 lightsabers in combat. He was able to asorb blaster bolts than channeling them into force lighting. He could save other from death but not himself.
Good list. But some of your info was a bit wrong,enough to have me respond to some of the errors.