When it comes to force sensitivity, you’re mainly going to be looking at two groups. The lightside and the darkside. While the lightside was nourished with mediation, understanding, and selflessness, the Dark Side was not. Fueled by hatred, fear, and vengeance the Dark Side could tap into the force, which in many cases, surpassed that of a Jedi.
Darth Tyranus (Dooku)
Dooku was a Jedi master and practiced as a Jedi for over 70 years. Eventually he slipped to the dark side after the battle on Galidraan. Dooku, who was one of the best swordsmen in the Jedi temple was now a dark side wielding badass who renamed himself to Darth Tyranus. He was later defeated by Anakin Skywalker.
“Skywalker was all over him. The shining blue lightsaber whirled and spat and every overhand chop crashed against Dooku’s defense with the unstoppable power of a meteor strike; the Sith Lord spent lavishly of his reserve of the Force merely to meet these attacks without being cut in half, and Skywalker was getting stronger. Each parry cost Dooku more power than he’d used to throw Kenobi across the room; each block aged him a decade. He decided he’d best revise his strategy once again. He no longer even tried to strike back. Force exhaustion began to close down his perceptions, drawing his consciousness back down to his physical form, trapping him within his own skull until he could barely even feel the contours of the room around him; he dimly sensed stairs at his back, stairs that led up to the entrance balcony. He retreated up them, using the higher ground for leverage, but Skywalker just kept on coming, tirelessly ferocious.”
Gethzerion was the clan mother of the Nightsisters. This force user was so powerful that she even had Palpatine shitting his pants at one point.
While the Nightsisters believed that they had to perform rituals to unleash their Force powers, only Gethzerion realized the true power that she could control. Emperor Palpatine knew of her power while he was alive and ordered that she be kept trapped on Dathomir where she could still rule but not use her power elsewhere. Zsinj, who would later become a ruling Imperial warlord, was assigned to keep an eye on Dathomir.
Later she clashed with Luke Skywalker and lost. Upon trying to flee Dathomir, Palpatine had her ship blasted to pieces.
Starkiller is an example of a Sith who used the force to it’s maximum potential, without caring what the ultimate consequence might be. As Vader’s apprentice he was sent out to kill the remaining Jedi, which he did with ease. When he was betrayed by Palpatine, things got ugly. Eventually Starkiller lost to Vader and Palpatine, but his defeat did not come quick or easy.
“Exar Kun, because of you, the Sith will never die. You have earned the right to bear the mark of Dark Lord of the Sith.”
―Marka Ragnos, appointing Exar Kun as Dark Lord
Kun ruled 4000 years before the Skywalkers roamed the galaxy. However, he managed to come back in spirit and almost put an end to Luke Skywalker and the NJO.
Thousands of years later, Exar Kun’s spirit returned to wreak havoc on the students of Jedi Master Luke Skywalker’s praxeum on Yavin 4. After haunting several of the trainees, Kun’s spirit subtly corrupted one of them, Gantoris, to the dark side of the Force, but killed Gantoris when he tried to reject the Dark Lord’s influence. He also took Master Skywalker’s soul from his body, but was ultimately defeated by the combined power of Luke’s students.
Darth Caedus (Jacen Solo)
Jacen Solo was the son of Han and Leia Solo and twin brother of Jaina Solo. His force powers as a Jedi peaked toward the end of the Yuuzhan Vong war. Later he had a vision of a dark figure ruling the galaxy. This vision changed all of his philosophies and he began dabbling on the dark side. He began his Sith apprenticeship under Lumiya, and shortly thereafter was at Sith mastery levels. He was a wrecking machine who was finally stopped by his twin sister.
Darth Vader (Anakin Skywalker)
Anakin Skywalker was foreseen by some in the council as the one who would bring balance to the force. Unfortunately his desire to keep his wife alive and lust for greater power, eventually turned him to the Dark Side. Once he became Palpatine’s apprentice he slaughtered anyone the defied him. His powers became so great that he could force choke someone wasn’t even in his vicinity. In the end he did fulfill the “chosen one” prophecy.
Refusing to watch his son die, Anakin suddenly grabbed Sidious from behind and hoisted him from his feet. In a final effort of self-sacrifice, Anakin stubbornly advanced, ignoring the agony his former master’s Force Lightning inflicted him as he went for the Death Star’s reactor shaft. Staggering to his destination, he threw Darth Sidious down the shaft to his death before collapsing. Mortally weakened by the lightning, and with his life support system short-circuited and fried, Anakin Skywalker had paid for his act of defiance with his life. Yet this single act brought him back to the light side.
Revan was a Jedi Knight who came into his prime during the Mandalorian Wars. Since the Jedi wouldn’t take an offensive stance on the battle, Revan made it his goal to win the war. To do this he had to be proactive, which eventually lead him to the Dark Side. Revan destroyed the Mandalorians, thwarted the Jedi, then went back to the lightside (after a memory wipe) and defeated the most powerful Sith at the time (his old apprentice Malak).
Nihilus was another victim who lost everything in the Mandalorian wars. The effect of a superweapon’s destruction left Nihilus craving the force. He was later found by Darth Traya who regarded him as “a scar in the force.” Nihilus became so obsessed with calling upon the DS that he could consume anyone or thing that had force sensitivity. His hatred and power was so great that even other Sith got the hell out of his way.
“There is no strength in the hunger he possesses… and the will behind his power is a primal thing. And it devours him as he devours others—his mere presence kills all around him, slowly, feeding him. He is already dead, it is simply a question of how many he kills before he falls.”
The Jedi had a prophecy claiming there would be “The One” who brought balance back to the force. The Sith also had a prophecy like this, only they used the word Sith’ari which meant “perfect being.” Darth Bane was the Sith’ari. At his physical peak, Bane was covered in a parasite called Orbalisks. Although making him look hideous, the orbs were able to deflect the brunt of any physical attack (even lightsabers) making him nearly invulnerable. He once slaughtered 8 sith assasins, a dark jedi, and defeated his apprentice at the same time. Some speculate that he was able to pull a moon out of orbit using the force.
Taming a flying Drexl using the powers of the dark side, he mounted it and rode towards the edge of the atmosphere where Dxun came close to Onderon’s atmosphere. Once on Onderon he found Zannah being threatened by a group of Beast Riders. Onboard the drexl, Bane attacked the Riders, slaughtering them and their mounts. Now they the two were together once more, it was time for Zannah to begin her training.
Darth Sideous (Palpatine)
Sideous used unmeasurable amounts of force manipulation to propel him in political office. Once there he created the beast known as the Empire. Billions died at the hands of (or direct influence) Palpatine. He was also responsible for turning Anakin Skywalker to the Dark Side.
Trained in the ways of the Force by Darth Plagueis, he was a Dark Lord of the Sith who followed the Rule of Two and was the most powerful Sith Lord the galaxy had ever known.