The dark side of the Force has been wielded and used for evil by not only some of Star Wars' best villains, but a couple of the most iconic baddies in pop culture. The true potential and nature of the dark side are not and likely never will be fully understood. However, fans can appreciate its power and how strong some characters are with it, whether that be in their knowledge of it or their sheer raw ability.
There is a vast disparity in power levels within dark side Force users. Some are simply nowhere near the level of others, particularly those true Sith Lords who utilize the mystical entity in such a terrifying way throughout the franchise.
Savage Opress
While Savage Opress was undoubtedly a monstrous character, a Dathomirian Zabrak with superhuman strength and explosiveness, his Force abilities pale in comparison to every other dark sider seen on screen.
Between three masters - Asajj Ventress, Count Dooku, and Maul - Savage never got anywhere near full training in the Force. He could utilize it to a certain level, but as seen in his limited training with Dooku and in his time with Maul, Savage had to rely more on his physical prowess.
The Inquisitorius
Thus far, fans have met four on-screen Inquisitors below the Grand Inquisitor; the Second Sister, the Fifth Brother, the Seventh Sister, and the Ninth Sister. All of them have shown themselves to be similar in terms of power.
They were never trained fully as Jedi, and when they fell to the dark side they were never trained fully as Sith. They were assassins who had experience with a lightsaber, a certain skill level in the Force, and that is it. The Inquisitorius are great characters that fans want to see more of, but in their Force abilities, they are far from overpowered in the canon.
The Grand Inquisitor
A former Jedi Temple Guard and character who proved to be a fantastic villain for Rebels' first season, the Grand Inquisitor, similar to the rest of Inquisitorius, was nowhere near the levels of those who trained him. Still, he did seem to be more powerful than the rest.
There is little evidence to say he was far and away the most powerful in the Force. Still, considering that he was a fully-fledged Jedi Temple Guard, killed Jedi, and was ranked higher than the rest of the Inquisitorius, it is safe to assume he was more talented in the dark side of the Force.
Asajj Ventress
Asajj Ventress has a fascinating journey through the Force in Star Wars. She was trained in the Force by a Jedi before falling to the dark side. She became a Sith assassin, followed by a bounty hunter, finally becoming an ally to the light side of the Force before her death.
As a dark side Force user, Ventress was always kept at an arm's length and never trained as a fully-fledged Sith. Throughout the Clone Wars, she grew in power, which prompted Sidious to demand her death from Dooku. Even Obi-Wan in Dark Disciple - a must-read for fans of the character - comments on Ventress' immense willpower and ability in the Force. She could have challenged the likes of the movie villains in power had she been adequately trained as an apprentice; by Sith or Jedi.
Maul
One character whom Ventress could have gotten close to in terms of power had she been trained like he was is Maul, arguably the best TV villain in Star Wars and a character raised in the dark side of the Force by Sidious.
The most significant piece of evidence to support Maul being a mighty Force user is his survival skills. Not only did he survive getting cut in half for a decade, but beyond that, on Malachor and his trek through Tatooine, hate, vengeance, and anger were fuel for him. Maul was skilled in all the typical Force abilities but preferred to use them conservatively. That was the case until his 'death' on Naboo prompted him to become more wild and free with it. Not only did Maul have the raw power, but he had experience and knowledge, which are tools as equally powerful as the potential itself.
Count Dooku
Knowledge and experience are great allies of Count Dooku, who benefited from decades of Jedi training at his disposal. The dark side gave him a new lease on the Force in which he was already incredibly powerful.
Dooku was exceptionally knowledgable in the arts of telekinesis and Force lightning, both of which made him one of the most dangerous villains in Star Wars when paired with his exquisite talent with a lightsaber. Even at his older age, Dooku's knowledge and skill in the Force allowed him not to get overwhelmed by younger, stronger warriors like Anakin - until Revenge of the Sith, of course.
Kylo Ren
To many who dislike the sequel trilogy, saying that Kylo Ren is more powerful in the Force than the likes of Maul and Dooku may be sacrilege. However, there is simply no denying the raw, untamed strength that the young Solo/Skywalker possessed in the dark side.
Kylo has main-character energy when it comes to his unlimited Force potential. As a Skywalker, he had extraordinary natural gifts. His focused rage, drawn from his constant internal struggle, had harrowing possibilities that could make him a threat to anyone. Gifted in telekinesis and telepathy, Ren could control others from afar and could invade people's minds with ease. Ren could also physically strengthen himself through the Force, among other things. His dyad with Rey only served to further his Force abilities - although he outmatched her in the Force during their tug-of-war on Pasaana.
Darth Vader
Darth Vader and his Sith Master are levels above any other on-screen dark side Force user in Star Wars, with the two being monstrously skilled and capable with the dark side of the Force.
Fuelled by pain, hatred, anger, passion, and suffering, Vader's grasp of the Force was as tight as the grasp he so often had around people's necks. Scary is too light of a word to describe how awe-inspiring Vader truly could be when wielding the Force. At times he is shown to be quite overpowered. Vader could lift AT-AT's above his head, rip starfighters from the sky, project Force barriers, crush internal organs, and more. Comic fans truly understand the vast knowledge and capabilities of Vader with the Force.
Darth Sidious
Vader/Anakin Skywalker had the potential to be far more powerful than the likes of Yoda and Darth Sidious in the Force. But the Skywalker story's events mean that Darth Sidious remains the strongest dark side Force user in canon; perhaps the strongest in the Force overall with the exceptions of entities like the Gods of Mortis, the Priestesses, and the Bendu.
Sidious is one of the most powerful beings in galactic history, without question. Everything mentioned that other Sith and dark siders were skilled in, Sidious was a master of, from telekinetic abilities to combative practices. Whether it be via Sith alchemy and magic or through using science, or even through his skill with using the Force to foresee the future, Sidious had every base covered. A Master in every sense of the word. He is not only the central villain of the Skywalker saga, but the best dark side lightsaber duelist, the most ingenious villain, and the most powerful with the dark side of the Force.