Star Wars has many characters strong with the Force, and Palpatine is easily among the most powerful. Also known as Darth Sidious, the politician-turned-emperor is a Sith Lord: a malevolent warrior who uses to Force for killing and conquest. These objectives make him the overarching villain of the Star Wars film saga. However, with that pedigree, though, he's not the top dog.
The Expanded Universe (also known as Legends) has seen countless Sith Lords throughout galactic history, several of whom are more powerful than Palpatine. They not only possess superior abilities, but have the lethal cunning to use those dark gifts to their full extent. They've all posed threats to the galaxy. More than that, they've shown what the Force was truly capable of.
7 Darth Plagueis
As implied in Revenge of the Sith, Darth Plagueis was Palpatine's mentor. That would be impressive enough, but his true strength lies in his uncanny ability. Namely, he can manipulate the Force to create life. By extension, he can prevent the deaths of those close to him.
True, his apprentice may have killed him, but this was only after he had lost much of his power. Even then, Palpatine had to snuff him out in his sleep. He obviously didn't want to take on his former master directly, which says a lot in itself. However, his cowardly action was premature; Palpatine has never been able to replicate Plagueis's unnatural gifts. Those under his thumb likely count their blessings because of that.
6 Darth Nihilus
While Plagueis has power over life, Nihilus has dominion over death. Emerging in Knights of the Old Republic 2, the masked figure was at ground zero of a cataclysmic planetary attack. Against all odds, his essence held on, absorbing the energy of all surrounding life forms.
Nihilus then roamed the galaxy, constantly searching for new warriors and worlds to consume. This earned him the title "Lord of Hunger." He's a devastating force of nature who targets both Jedi and Sith. He doesn't distinguish between the two, and he inspires fear in both factions as a result.
5 Darth Traya
The only KOTOR 2 character more dangerous than Nihilus is his former master. Kreia has spent time as both a Jedi and a Sith, using her knowledge to instruct prominent figures like Nihilus and Darth Sion. Of course, this has equipped her with life draining and other dark abilities. Nevertheless, she stays in the middle. Though Palpatine claims to embrace the Force "in all its aspects," Kreia truly embodies that philosophy. This makes her unpredictable since she doesn't conform to conventional morality. That further stems from her immense intellect.
Kreia easily matches Palpatine in her Machiavellian machinations. Her schemes deceive both the Jedi and the Sith, pitting them against each other and letting her mop up the rest. Even her defeat is part of that plan, as she engineers it all to prepare her new apprentice for a later battle. When Kreia sets a goal, she goes to any lengths to achieve it.
4 The Exile
Kreia's greatest contribution is the player character. Though gamers can choose their gender, the Expanded Universe canon describes the protagonist as a woman named Meetra Surik. Losing her allies in the Sith war, this Jedi Knight suffers a loss so catastrophic that it throws her Force energy out of balance. This essentially makes her presence a gaping hole in the Force, one which sucks in surrounding life.
The more opponents she defeats, the more life she siphons, and the stronger she gets. Her abnormal aura is also the crux of the Force bonds she forms. She reveals the hidden strengths of her party while dominating their will and compelling them to follow her. It's an unconscious mix of admiration and revulsion. All of this could eventually lead to the death of the Force, an outcome which is more likely if players pursue a darker path. No other Sith Lord has ever posed such a threat to the primordial fabric of the universe.
3 Darth Krayt
No, he's not a dragon. More than most, this Sith Lord exemplifies raw power. As a young Jedi in training, his rage lets him topple threats that even his masters can't overcome. That strength only amplifies once he turns to the Dark Side. For instance, his lightning doesn't just shock enemies; it reduces solid stone to rubble. Of course, such power creates a ripple effect.
Krayt's influence stretches across time and space. Other Sith can sense his might from lightyears away. On top of that, he lives for centuries, even returning from his supposed death in battle. His foresight only complements these gifts. Seeing the future lets him set plans in the longest terms, and his lifespan ensures he'll be there to carry them out with little resistance.
2 Darth Bane
Here's a marriage of knowledge and might. Bane was exceptionally gifted from a young age, excelling past many students at the Sith academy. He fuels that natural talent with the swath of scrolls and texts collected over the years, making the most of already-deadly techniques. Killing instantly with Force Lightning and creating a widespread life-draining field are just a couple of examples. However, these are only his offensive moves.
His defense is every bit as effective. Bane dons specialized orbalisk armor which is impervious to lightsabers. In addition, it chemically enhances his strength, stamina, and senses. Many aspects of Star Wars fall into categories of science or sorcery, but Bane combines the two with terrible results.
1 Exar Kun
What Darth Bane accomplishes with science, Exar Kun pulls off through sheer Force mastery. His is a familiar story of a prize pupil who goes bad. He turns to the Dark Side for the possibilities it offers, and he takes ample advantage of those possibilities. On top of his innovation of the double-bladed lightsaber, Kun's battle rage enhances his physical prowess beyond even other Force users, letting him shatter solid objects with a single punch. As shocking as such feats are, the Sith Lord's gifts go beyond simple combat.
Kun's expertise and creativity birth all sorts of dirty tricks. He can target internal organs and destroy them using the Force, and he creates a plethora of biological monstrosities through Sith alchemy. Perhaps most useful, though, is his mental domination. He influences an entire senate with a blanket mind trick, forcing them to preach Sith teachings. This ability even works on Jedi; he saps their free will and sends them to kill their masters. Years of manipulation is circumvented with just a flick of the wrist.
Granted, the villain doesn't really need the time. His affinity also enables him to ascend to a spiritual existence, leaving his own body behind to wreak further havoc. Light Side users thought they had the monopoly on this with Force Ghosts, but Kun thumbs his nose at them all the way to the grave and beyond. With him, it seems the Dark Side has no downside.