Since Alleria has her void powers after eating a Dark Naru, it means she also has a "thousand years" of fighting the most powerful force in the cosmos up to the Void Lords, which are still in the background. And if the Void is the strongest, then that still makes Alleria top dog as she became one with it. It wouldn't be a contest, even if Sylvanas had home field advantage and her entire dark rangers at her beck and call. Doubt even her blight would make a difference. Though that's just a theory based on what we know to be true, with a lot of unknowns regarding how strong a Dark Naru is and how much control she has over that immense power in her possession.