Darth Sidious
Sidious is also known as The Emperor of the Galactic Empire. He rose to power in the Senate as a Senator from Naboo and lived a double life as a Sith Lord. Sidious is extremely powerful and held very complex knowledge of techniques to master the dark side's force energy.
Yet, Darth Gravid was far, far different in many respects. He was one of the few Sith Lords to show actual respect for his Jedi opponents. Gravid was a powerful Sith Lord dedicated to his cause. However, he also believed that strict adherence to the dark side would be the Order's eventual undoing.
1. Darth Sidious (Star Wars: Episode I – The Phantom Menace) Truly, the most powerful Sith Lord was Darth Sidious, better known in his public persona of Chancellor (later Emperor) Palpatine. Through cunning and manipulation, Sidious killed his master to claim the mantle of Dark Lord of the Sith.
Darth Vectivus was created by Aaron Allston for the first novel of the Star Wars: Legacy of the Force series, Legacy of the Force: Betrayal. He appeared as a spirit to Nelani Dinn and served as a platform for Lumiya to convince Jacen Solo to become Darth Caedus by showing him that not all Sith were necessarily evil.
That said, Vitiate, sometimes better known as the Sith Emperor, is often thought to have been stronger than Darth Sidious thanks to his mastery of the Dark Side of the force which granted him a ridiculously long life.
According to Star Wars Legends content, the first Sith was a former Jedi Master, Ajunta Pall, who became a so-called Dark Jedi, or “Fallen” Jedi.
If it came to strategic prowess then most likely you'd find Sidious ahead. He was the one after all who managed to outsmart the entirety of the Jedi Order and Republic (minus the few individuals aiding him). And was then smart enough to keep any rivals in line or at a distance during his reign as emperor.
As the first Dark Lord of the Sith, Ajunta Pall founded the first Sith Empire and expanded it onto other worlds. The Sith took over the planet Ziost and created it as their capital and as Pall's new home.
Now, Darth Vitiate lived for around 1,500 years, which is by far the longest lifespan of any Sith Lord on this list. With a major appearance in Star Wars: The Old Republic, Darth Vitiate presented himself as one of the big bads in the Star Wars universe.
Jedi like Luke Skywalker and Obi-Wan Kenobi have long stood as light against the darkness. But the record Sith-killer is actually Darth Bane himself. Darth Bane, founder of the Rule of Two, killed more Sith than any Jedi in the history of the Star Wars galaxy.
Darth Vader Kills Starkiller (Star Wars The Force Unleashed) - YouTube.
In Star Wars Legends and Canon, Galen Marek (Starkiller) is the only Jedi (other than Obi-Wan Kenobi) to put a beat down on Darth Vader/Anakin Skywalker. Galen was secretly trained as a Sith Lord apprentice to Vader, but returned to the light side and the Jedi.
Emperor Vitiate was a Sith Lord who successfully gained near immortality by draining the life force of an entire planet, later attempting to drain all life in the galaxy, which would give him true immortality.
Without question, the most powerful Force ability present in Star Wars is the Thought Bomb. It is created through an ancient Sith ritual where the Sith combine their dark side powers. If done successfully, the Thought Bomb can destroy all Force-sensitive beings unfortunate enough to be within the radius of its blast.
Kenobi and Anakin Skywalker (and later Darth Vader) in the upper part of this range, 130–140.
With the death of T'ra Saa at Taivas, K'Kruhk became the oldest and most experienced of the surviving Jedi and participated in the alliance's final assault on the Sith-held Coruscant. Following the deaths of Darth Krayt and Roan Fel, K'Kruhk became one of the three Triumvirs of the restored Galactic Alliance.
Sidious, born Sheev Palpatine, was trained in the dark side of the Force by Darth Plagueis.
Within the Star Wars Legends continuity, Yoda's Jedi instructor was N'Kata Del Gormo, a Hysalrian Jedi Master, according to an in-universe legend.
Anakin Skywalker could have thwarted Darth Sidious and in fact did, albeit almost 25 years after he was given the initial opportunity. Anakin fell into Sidious' trap and attacked Windu, as Sidious played victim, only to then spring to life and fire lightning upon Windu.
One thing that stands out is the fact that Anakin was weaker than Vader specifically against Obi-Wan, not necessarily in general. Anakin's emotions cost him the fight with Obi-wan, as he did exactly what Obi-wan wanted.
After a lightsaber duel against Maul and Savage, Sidious easily kills the latter, enraging Maul, who briefly holds his own against Sidious, but is ultimately defeated. Sidious proceeds to torture Maul with Force lightning, but spares his life because he has other uses for him.