Lord Fulminiss was the greatest Sith alchemist in the entire Empire:

"They are the only surviving work of the Empire's greatest alchemist, Lord Fulminiss."
- Darth Acina

He's among the Emperor's greatest servants:

"Lord Fulminiss. The Emperor isn't taking any chances with Voss."
- Lord Scourge, SWTOR

Considered to weild incredible power:

Widely regarded as one of the greatest Sith sorcerers in the Empire, the enigmatic Lord Fulminiss is a master of the dark side’s most arcane and lethal aspects. Legend has it he once summoned a raging storm of pure Force energy that disintegrated a rebellious city of natives in the Imperial-conquered Jabiim system.
- Lord Fulminiss Codex Entry

His alchemical power was immense:

One of the greatest practitioners of Sith alchemy, Lord Fulminiss, devoted his life to developing insidious new devices for corrupting other life forms. The Seeds use a combination of low-frequency transmissions and pure dark side power to subtly manipulate the thoughts and emotions of any living creature nearby. Over time, victims of these devices are driven homicidally insane and contaminated by dark side energy.
- Seeds of Rage Codex Entry
"The Dread Masters' agents are burying Seeds across the galaxy. Where they will drive entire populations mad."
- Darth Acina

Per Sel-Makor, Fulminiss was less powerful than Darth Baras as of the Great Galactic War; prior to Baras being gifted a great boon of power:

"Rewards. Power. Sel-Makor offers them to you. Like the one who came before."

"Who are you talking about? Was there someone here before Fulminiss?"

"Decades ago. A Sith. Greater than Fulminiss. Power was given."
- Sel-Makor and the Battlemaster of the Jedi order

Per Grand Master Satele Shan, Lord Malgus as of the Battle of Korriban was the single most dangerous embodiment of the dark side she'd ever met; including Darth Baras:

"There in the hangars I encountered the most dangerous embodiment of the dark side I have ever experienced-the man who would later be known as Darth Malgus."
- Satele Shan, The Journal of Master Gnost-Dural[/quote]

Battlemaster Kao Cen Darach was capable of blasting back Lord Malgus a great length as depicted in this behind-the-scenes shot:

https://static.comicvine.com/uploads/original/11117/111178634/5684746-tk+hurls+malgus.gif[/img]

As a Battlemaster, Kao enjoys the following statement:

He's one of the best Jedi of his time:

He is however overshadowed by Battlemaster Ven Zallow, who shares both of the aforementioned accolades, as well as this:

[QUOTE]MOST FAMOUS KILL:
JEDI VEN ZALLOW
- Darth Malgus Bio, Holonet, Star Wars the Old Republic Website

Which is evident here, where Zallow matches a far more powerful iteration of Darth Malgus than Kao Cen Darach was killed by:

Their blades sizzling in opposition, each used the Force to press against the strength of the other, but neither had an obvious advantage.
- Deceived

This is incredibly impressive, as Darth Malgus has feats such as matching the thruster power of a freighter:

He reached out with the Force as Razor continued its rise, tried to take it in his mental grasp. Its ascent slowed. He held forth both of his arms, made claws of his hands, and shouted with frustration as he sought to hold back the power of the ship's thrusters.

He felt a tightness in his mind, the string of his power being drawn taut, stretching, stretching. He would not release the ship. Its thrusters began to whine. He held it, teeth gritted, sweat soaking his body, his breath a dry rattle through his respirator.
- Decieved

He's also stated to have achieved battlefield feats as a Sith warrior that have not since been duplicated:

Malgus submitted utterly to the dark side, and doing so made him an exemplary warrior. His battlefield feats have never been duplicated.
- Darth Sidious, Book of Sith

He's one of the most powerful Sith the Galactic Republic would ever know:

In Deceived, you’ll learn the story of Darth Malgus and his rise to power. Delve into the dark secrets and history that shaped one of the most powerful Sith Lords the Old Republic would ever know…
- Deceived Novel Trailer, swtor.com

He's further stated to rival even the 'most sinister' of the Dark Lords of the Sith:

The second novel set in the Old Republic era and based on the massively multiplayer online game Star Wars: The Old Republic, ramps up the action and brings readers face-to-face for the first time with a Sith warrior to rival the most sinister of the Order's Dark Lords—Darth Malgus, the mysterious, masked Sith of the wildly popular Deceived and Hope game trailers.
- Decieved inner flap

Ahead of Ven Zallow would be Master Warren Sedoru:

"Warren was the Order's greatest soldier in his day--"
- Tol Braga, Star Wars the Old Republic

Warren's among the strongest Jedi of the Order:

With Grand Master Satele Shan's support, Master Braga assembles a strike team of the strongest and most resolute Jedi in the order.
- Star Wars the Old Republic: Encyclopedia

Who is surpassed by Jaric Kaedan, a Senior Jedi High Council member who survived a Dread Masters encounter:

One of the senior members of the Jedi High Council, Master Jaric Kaedan was born in the Outer Rim and, although he wasn’t discovered by the Jedi until he was twelve, quickly rose in prominence. During the war, Master Jaric Kaedan joined the Republic forces fighting on the front lines and was involved in several notable military actions, including the battles on Rhen Var and the capture of the Dread Masters.
- Master Jaric Kaedan

Kaedan is surpassed by perhaps the greatest Jedi of the era, Master Wyellett:

"The lost super weapon Xerender seeks is actually a Jedi Master named Wyellett. Before his capture during the war, Wyellett was among the Jedi's most powerful Masters."
- Darth Baras, Sith Warrior ACT II, Star Wars the Old Republic
"Here, I communed with the Force to the expense of all else. It has been a great privilege and quite mind-expanding. The Force is with me, greater than ever now."
―Master Wyellett, Sith Warrior ACT II, Star Wars the Old Republic

Wyellett was already an incredibly powerful Jedi master during the Great Galactic War. Even surviving the destruction of a ship, its descent through orbit as wreckage, and its crash. Prior to spending more than twelve years growing far more powerful:

Jedi Master Wyellett was one of the great Jedi heroes of the last war. The Imperial starship that was transporting Wyellett was destroyed in the Battle of Hoth, and Master Wyellett has been believed dead all these years. In fact, Wyellett has been in a trance beneath the rubble of the fallen ship, psychically trying to reach out to his former pupil. Now, Wyellett has transcended the concerns of this galaxy and his powers are more realized than ever before.
―Master Wyellett Codex Entry

Wyellett is slain by the Emperor's Wrath, who goes on to face the even more powerful Darth Vengean:

"How would you like to face off against one of the twelve most powerful Sith Lords?"
―Darth Baras to his apprentice
All Sith have their masters. The one commanding Darth Baras is the Dark Council’s feared military leader, Darth Vengean. Vengean was openly outraged when the Treaty of Coruscant was signed into law, criticizing all who supported the peace accord and even quietly disparaging the Emperor. That he survived such insolence is a testament to his power.
- Darth Vengean Codex Entry

Yet he is not as powerful as Darth Ekkage; sister of Darth Baras, per the Emperor's Wrath:

"I've never faced a stronger foe."
- Emperor's Wrath to Darth Ekkage

She is among the most powerful Sith assassins ever, considered as powerful a weapon as any Sith has ever been:

The sister of Darth Baras, Darth Ekkage is one of the greatest Sith assassins to ever live.
Many believe Darth Ekkage was as powerful a weapon as the Sith have ever had.
―Darth Ekkage Codex Entry

Darth Baras himself, of course, is much more powerful:

"It is time for you to confront Darth Baras. Even now, Baras is near-indestructible."
- Darth Vowrawn

He can shake the Dark Citadel in Kaas City, Dromund Kaas with his anger:

He's stated to have been draining power from Darth Traya's spirit for years:

"He feeds off this spirit's power, stealing all her visions of the future. Everything he has built has come from her insights."
- Darth Vowrawn

Despite this monumental power, Baras is destroyed by the Emperor's Wrath:

The Wrath battles Darth Baras in a fierce duel. The Wrath emerges triumphant, striking down the ruined and seething Darth Baras.
- The Old Republic Encyclopedia

The Emperor's Wrath then goes on to ally against Malgus, the Dread Masters, Revan, Arcann, Vaylin and even Tenebrae. Getting more powerful with each conflict. This should show that SWTOR is home to a many great powerhouses.