There has been no Sith, as powerful, meticulous, nor as evil as the Emperor himself, Darth Sidious. As previously mentioned, Sidious trained in the Dark Side by Darth Plagueis before Sidious killed Plagueis in his sleep, making Sidious a Sith Master.
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.
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.
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.
Darth Shain, headmaster of the Korriban Temple, took notice and convened with the Dark Councilor and Dark Lord of the Sith Darth Shane McMahon, who decided to confer the title of Sith Lord to the young apprentice at the age of 11, making him the youngest Sith Lord in the history of the Sith.
By far the most iconic Sith Lord and warrior is Darth Vader (or Anakin Skywalker).
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.
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.
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.
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.
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.
A Sith Lord with the ability to cheat death and create life, Plagueis is the mentor of Sheev Palpatine / Darth Sidious, who, in accordance to the Sith's Rule of Two, eventually betrays Plagueis by murdering him in his sleep, taking his place as Sith Master.
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.
Kenobi and Anakin Skywalker (and later Darth Vader) in the upper part of this range, 130–140.
Darth Sidious (Sheev Palpatine) was a human Dark Lord of the Sith who appeared in each trilogy of the Skywalker Saga.
Emperor Palpatine / Darth Sidious.
1 Luke Skywalker
Having faced his father once before, Luke was familiar with his tactics and used the force to gain an upper-hand. In Star Wars Canon, only Luke Skywalker can be acknowledged as the one who truly defeated Darth Vader.
Tenebrae—known to the Old Sith Empire as Darth Vitiate, his reconstituted Sith Empire as the Sith Emperor, and his Eternal Empire as the Immortal Emperor, Valkorion—was an ancient dark-side entity of enormous power who manipulated galactic affairs for nearly 1,500 years.
Six famous Sith Lords bearing the Darth title, from left to right: Darth Andeddu, Darth Revan, Darth Malak, Darth Malgus, Darth Ruin, Darth Bane. Many Sith Lords chose to add "Darth" to their name, so much so that the word is considered synonymous with the dark side of the Force in some circles.
The decree commanded that only two Sith Lords must exist at any given time: a master to represent the power of the dark side of the Force, and an apprentice to crave it and train under the master and to one day fulfill their role.
Over the next decade, the galaxy was on the verge of civil war with entire star systems—led by Count Dooku, Yoda's one-time Padawan—threatening to secede from the Republic by 22 BBY.
Padawan Windu was trained in the ways of the Force by Jedi Master Cyslin Myr.