Originally Posted by
Varodoc
The latest cinematic confirms that Alleria is stronger than Sylvanas and is the most powerful Windrunner in the franchise.
Tyrande with the power of Elune was able to completely defeat Sylvanas and would have killed her if the power didn't run out. Alleria absorbed the power of L'ura, a Void Naaru. In Legion it was revealed that the Naaru are created by and infused with Elune's power just like Tyrande. This means that Alleria, who is infused with the Naaru's and thus by proxy Elune's power, would easily overwhelm Sylvanas.
If the two fought in Three Sisters comic, Alleria would have stomped her evil sister.