Registered: Jun 2016
Location: The Throne of the Sheevites
Valkorion is outclassed in every single aspect there is to compare.
Accolades: Being stated as the most powerful Sith Lord and dark side user time and time again >>> anything Valkorion has
Telekinesis: Ragdolling a 19 km Super Star Destroyer >>> anything Valkorion has done
Force Lightning: Bending lightsaber blades, damaging lightsaber-resistant objects, using Lightning across galaxy-wide distances >>> anything Valkorion has done
Telepathy: Casually dominating 20 billion people from across the galaxy, casually maintaining galaxy-wide Battle Meditation >>> anything Valkorion has done
Force Drain: Casually draining 20 billion people from across the galaxy >>> anything Valkorion has done
Force Storm: Ripping off the surfaces of planets, being capable of annihilating moons >>> anything Valkorion has done
Cosmic accomplishments: Turning the galaxy into a dark side nexus with your mere presence, clouding both sides of the Force, creating galaxy-wide shockwaves with a fraction of your power >>> anything Valkorion has done
Sith Alchemy and Sorcery: Creating rituals that caused galaxy-wide ripples in the Force, learning and studying canonically the greatest facets of Sith Alchemy and Sorcery >>> anything Valkorion has done
Force Knowledge: Gathering Force-related works from over a million worlds, studying all aspects of the Force including the Jedi's and other obscure Force cults, knowing canonically the greatest dark side lore, studying the dark side in greater depth than anyone else before him, knowing dark side abilities previously unknown to anyone else in history >>> anything Valkorion knows
Success: Conquering the galaxy >>> failing to do so
Impact on pop culture: Being considered among the greatest villains in cinematic history >>> being known only among the hardcore geeks and nerds
Registered: Jun 2016
Location: The Throne of the Sheevites
We know he telekinetically buried the Lusankya, and we also know he did this with a fraction of his power, as at the same time, he was clouding the minds of about one trillion people, dominating and draining the 20 billion populace of Byss, and providing Battle Meditation to tens of trillions of Imperial troops all over the galaxy.
If he is focusing purely on TK, and on nothing elss like he was with the ship's burial, he could toss those SSD's like tennis balls.
All you really need are the various statements saying that Sidious is the most powerful dark side user ever. That alone puts him above Valkorion. Sidious would basically just inherit, in a way, any of Valkorion's feats that happen to be superior.
To be fair you could argue for vitiate success wise given he was able to live for millennia with delaying death arguably being the main objective of the sith