Marka Ragnos is stated to be "the most powerful of the most powerful", making him the most powerful Sith Lord up to and of his time. This makes him more powerful than Ludo Kressh and Naga Sadow as of his funeral, but also Karness Muur and Tulak Hord:
Marka Ragnos ruled the galaxy with an iron fist. He was the Dark Lord of the Sith — the most powerful of the most powerful. But now he is dead.
Source: Tales of the Jedi: The Golden Age of the Sith 2: Funeral for a Dark Lord (Publisher's Summary)
Marka Ragnos is the greatest Dark Lord of the Sith in Old Republic times:
Meanwhile, on the far side of the galaxy, the Sith Empire has grown powerful through centuries of dark Force wielding and magic and the hundred-year rule of the greatest Dark Lord of the Sith, MARKA RAGNOS.
Source: Chronicles of the Old Republic
He is one of the most powerful and legendary Sith Lords in Old Republic history:
Sith acolytes spill blood and prove their strength in the mausoleums of history's most powerful and legendary Sith Lords.
Source: The Old Republic Encyclopedia
Marka Ragnos is one of the most powerful Sith Lords in history:
The most powerful Sith Lords possess a talent for the dark alchemy of the Sith. The Emperor, like Naga Sadow and Exar Kun long before him, became a master of manipulating the genetic building blocks of living creatures through the dark side.
Source: The Dark Side Sourcebook
Marka Ragnos is a master of the dark side of the Force:
The ruling Dark Lord of the Sith is a master of the dark side of the Force.
Source: Knights of the Old Republic Campaign Guide
Finally, a Sith Lord arose who exemplified the virtues of the Sith. Marka Ragnos was an imposing, merciless figure, a master of the Force and the arts of war.
Source: The Old Republic Encyclopedia
Ragnos's rule as a Dark Lord was marked by power, strength and strategy:
Strength, power and strategy marked the rule of Marka Ragnos.
Source: The Old Republic: Codex Entry: The Tomb of Marka Ragnos
Ragnos possesses a frightening grasp of the dark side of the Force:
Ragnos was a half-breed Sith, a warlord of tremendous physical power and a frightening grasp of the dark side of the Force.
Source: The Dark Side Sourcebook
Marka Ragnos possesses tremendous strength in the Force. The game dialogue files also show that Kreia is in awe of his power:
{barely detectable awe}
“Before you is the tomb of the great Sith Lord Marka Ragnos, a half-breed who possessed tremendous strength, both physically and in the Force."
Source: Knights of the Old Republic II: The Sith Lords
Luke Skywalker claims that it will require all of the strength of the New Jedi Order to stop a resurrected Ragnos:
"If Ragnos is resurrected, there's no telling what he might be able to do. It will take all of our strength to stop him."
Source: Jedi Knight: Jedi Academy
Emperor Vitiate regards Marka Ragnos's spirit as powerful enough to pose a threat to him when he returns to the known galaxy:
The Emperor knew the spirits of the ancient Sith Lords could be a threat to his power when he returned to the known galaxy a century later…
Source: Timeline 7: Peace for the Republic
Satele Shan and Darth Marr are able to sense the spirit of Marka Ragnos (along with the other ancient Sith) on Yavin IV:
"You sense it too. Not the ancient Sith, or the Emperor. Not Revan. Another presence... different from the rest."
Source: The Old Republic: Shadow of Revan
Telekinesis:
Marka Ragnos's spirit is able to hurl back the likes of Darth Nox, even when the latter is strengthened by a nexus as potent as the likes of Yavin IV
Drain:
Ragnos is capable of siphoning the life energies of others:
[Marka Ragnos is] able to draw life energy from living beings, gradually absorbing it [himself].
Source: The Dark Side Sourcebook
As a member of the ancient Sith, Ragnos mastered the ability to drain a being of its life force. It is additionally implied that Ragnos knew a ritual similar to the one the Sith Emperor used to devastate Nathema:
Through their focus on these elements of the Force, the Sith have developed terrifying powers, such as the ability to drain a being of its life force, or unleash their hatred as crackling bolts of energy. The Sith Emperor, history's most powerful dark side master, performed a ritual of incredible scope to consume the life energy of every being on his homeworld. Ancient Sith were also masters of these arts - Lord Naga Sadow was able to affect the stars themselves, and caused the red supergiant Primus Goluud to go nova.
Source: The Old Republic Encyclopedia
Lightning:
Ragnos is able to unleash his hatred as "crackling bolts of energy", which is likely Force Lightning:
Through their focus on these elements of the Force, the Sith have developed terrifying powers, such as the ability to drain a being of its life force, or unleash their hatred as crackling bolts of energy. The Sith Emperor, history's most powerful dark side master, performed a ritual of incredible scope to consume the life energy of every being on his homeworld. Ancient Sith were also masters of these arts - Lord Naga Sadow was able to affect the stars themselves, and caused the red supergiant Primus Goluud to go nova.
Source: The Old Republic Encyclopedia
Sense:
Marka Ragnos senses Vitiate's power and potential:
With all of Medriaas under his power, Tenebrae traveled to Korriban and met Sith lord Marka Ragnos, ruler of all Sith. Marka Ragnos recognized Tenebrae's incredible power and named the young boy Lord Vitiate.
Source: The Old Republic Encyclopedia