Nyriss most likely; she have taken on and subdued two heavyweights simultaneously without issue. Zannah doesn't matches this level of performance, skill and ferocity.
Nyriss is also proficient in the use of Sith Sorcery; not much is known about her knowledge in this field at the moment but "canon assumption" is that she knows a lot. Therefore, Nyriss may have some surprises of her own with her Sith Sorcery talent.
Zannah's only chance is with her Sith Sorcery talent but Nyriss is damn fast. Over-all, Nyriss is superior and have a good chance at overwhelming Zannah early on.
I say Nyriss would win.
Force: Toss up
Zannah's Tendrils vs Nyriss' lightning
I don't know if Zannah knows tutaminis. Nyriss can charge up her force lightning pretty quickly, but Zannah has to concentrate a lot to summon the tendrils. I think the tendrils are more devastating overall, but the question is can Zannah last long enough to conjure them before Nyriss turns her to dust?
Whether Nyriss can defend against Zannah's sorcery is an unknown. If she can, then I would say that gives Nyriss an even bigger advantage. If she cannot, then things may be more even or tip in Zannah's favor.
Sabers: Nyriss
Zannah has trouble mounting offensives; Nyriss soloed Scourge and the Exile.
Because there is no definitive mapping of skill/power from the KotOR era to the TOR era, to the Bane era, to the movie era, etc, it's hard to say how well Zannah would be able to hold off Nyriss. I don't think she would last long though.
Overall: I'm leaning towards Nyriss.
I'd say Nyriss. Casually owning the likes of Scourge and the Exile [who imo is considerably powerful] showcases her amazing skill and combat prowess. I doubt Zannah would be able to own those two as easily as Nyriss did. She turned humans into charred husks instantaneously with her lightning, and her skills with a lightsaber are enough to beat these two simultaneously, as well as overwhelming them with the Force. Zannah's good, just not as good as her.
Originally posted by NewGuy01
She wasn't taken out of the fight, she was just knocked off her feet. And that bolt had caught her in the chest. I doubt Zannah couldn't stop it with her lightsaber, considering she stopped Bane's nexus-amped lightning easily.
She was out of the fight in that she was unable to do anything while Nyriss was free to charge up her attack and gloat.
I also think Zannah will be able to block it, although Banes lightning wasn't nexus-amped.