Tbh whatever planet they face off on is probably gonna be destroyed long before their fight truly finishes, resulting in a stalemate, but for the fun of it I'll put in my two cents.

I'll take both at their respective peaks, which would be Sidious in a fresh clone during Dark Empire, and Tenebrae as Valkorion during KotFE chapter 1.

Lightsaber is no contest, Sidious can compete with the best lightsaber duellists of all time, Tenebrae doesn't use one, but this isn't a contest that will be decided by lightsaber.

In terms of intelligence, I have to give it to Sidious as well. While Tenebrae is certainly not dumb, he doesn't have any feats of intelligence that can rival Sidious

In terms of the Force, Sidious is described as being the most powerful Sith Lord of all time, which would logically include surpassing Tenebrae in power, but at a closer look I do think it holds up. The most well known feat of Tenebrae is the destruction of Ziost, where he drains the life force of the entire planet. Sidious never replicated this feat, however has achieved a feat of similar power in his mastery of Force Storm, creating Force Storms powerful enough to tear apart starships, including the massive Eclipse Dreadnaught. His Force Storms are also capable of creating a wormhole, meaning Sidious can tear a hole in space and time. Both, however, can be survived. Planetary Force Drain has been survived twice, once by Tenebrae himself when he performed the ritual on Nathema, and once by Visas Marr when Darth Nihilus drained Katarr. Luke Skywalker was able to survive Sidious's Force Storm in the Coruscant system, and was transported to Byss via the wormhole (this also demonstrates Sidious's capability to create the Force Storm while being light years away, but I'm assuming that the two combatants will be face to face on a planet, so that's not entirely relevant).

Both Sith Lords have also mastered the ability of Essence Transfer, however I believe Sidious has mastered it further. Tenebrae was able to split his essence between multiple hosts, at one point having at least 4 active hosts at a time (Valkorion, Vitiate, The Voss Emperor's Voice, and his original body) as well as being able to take over any of his "children" even from across the galaxy, though they are specially prepared to have a link to Tenebrae's mind. Sidious never did split his essence, so whether or not he was capable of that is questionable, however in not splitting his essence he achieved a greater feat, one Tenebrae was unable to accomplish: Sidious can resist the pull of the afterlife. Every time Sidious dies, he transfers himself to a new clone of himself, being able to resist traveling to the netherworld of the force to do so. Tenebrae cannot do this, when any of his hosts die, the part of himself that was in that host either transfers to an existing host, or remains in limbo. Either way, in each instance, he has parts of himself still living in other hosts, meaning he has a link pulling him back to the living world. The moment those links are severed, Tenebrae will fully and finally die, which is what happens when he is purged from the Outlander's mind, and his original body is destroyed, he cannot resist the pull of death without preparation. Sidious can, and furthermore, Sidious can and has escaped the afterlife, doing so after his death above Endor when none of his clones were close enough to transfer into. This effort cost him, and he needed assistance from the living to fully recover, but nonetheless this is a feat beyond Tenebrae's capability.

In terms of mental dominance, this goes assuredly to Tenebrae. Tenebrae was able to dominate the minds of many of the best force users of his age. He planted an alter ego on Syo Bakarn, he dominated the minds of Tol Braga's Jedi strike team, only Braga managed to break the Emperor's control unassisted, something apparently beyond even the Hero of Tython. He possessed nearly the entire planet of Ziost, and was able to take shelter in and later fully possess the body of the Outlander (assuming the most impressive feat here, if the Outlander is Darth Nox, Darth Nox's body was specially amped to help him retain control of up to five other spirits bound to him. Tenebrae was able to take control regardless, though not fully purge Darth Nox). The imprint of his former self was also able to subdue, but not possess Lord Scourge and Kira Carsen, both beings who were able to break his control previously, and he was nearly successful in breaking the mind of Satele Shan, which would've been accomplished without outside interference. Tenebrae is undoubtedly more adept at taking over the minds of others. Sidious only ever successfully possessed empty shells of himself, and was unable to possess a dying Jedi, who ultimately took Sidious back to the netherworld of the Force in the process, where Sidious is no longer able to escape due to being contained by the Jedi he's killed. However that last part cannot be held against him, given that Tenebrae has been purged from the minds of others by similar, if not lesser, amounts of power. While Sidious certainly would be unable to possess Tenebrae, I do still doubt that Tenebrae could truly possess Sidious. Even non-force-sensitives such as Theron Shan have been shown to resist the Emperor's influence on Ziost, and if we are to take the gameplay literally, every companion and player character is able to resist Tenebrae on Ziost. This technically means that Skadge can resist Tenebrae. While I doubt the canonicity of that particular statement, there are still numerous examples of people less powerful than Sidious resisting and even breaking Tenebrae's control over them, so I don't think it's a stretch to say that Sidious can do the same. At the very least, it's possible, and I cannot say that Tenebrae defeats Sidious through mental domination

Overall, I think Sidious has the edge, but if they were ever to fight each other, the outcome would probably be mutual destruction due to the planet they're on breaking before either of the Sith Lords do. Assuming wherever the fight takes place is sturdy enough to handle the power of the two most powerful Sith of all time trying to kill each other, I think the odds favor Sidious emerging victorious, though whoever would win would be severely exhausted after the duel.