Vader. Sylvanas' weapons are arrows and blades, the former can be thrown back with the Force and the latter wouldn't stand a chance against a Lightsaber. The only thing that she MIGHT be able to do is fight in Banshee form. We don't know if Vader can hurt ethereal beings, and I don't think he can withstand a Banshee scream. Maybe he has some sound-dampening in that helmet.
Vader, as Anakin, literally bested what were essentially beings made of the the Force ... I think he could hurt Sylvanas in her Banshee form. And the Banshee scream isn't something too far from powers that existed in Legends. Arguably, Sylvanas compares pretty well to a Nightsister.
Peace is a lie. There is only passion. Through passion I gain strength. Through strength I gain power.
Through power I gain victory. Through victory my chains are broken. The Force shall set me free.
–The Sith Code
This doesn't really feel relevant to this forum, and I can't really think of anywhere it could otherwise go. Closing this.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead