Vader saves Luke, taking lightning from Palpatine in the process. This lightning was intended to kill, so it likely wasn't casual. Palpatine is currently rated as High 4-C, so I feel confident in saying that Vader should be raised to this level, and those capable of standing up to him in direct combat should also be raised to this level (most notably Luke and Starkiller)
Also if Starkiller and Luke were scaled to this then it would make no sense as Palpatine overwhelmed them effortlessly. The explosion that killed Galen left Sidious completely unharmed and Luke couldn't even hold off his lightning for a second before writhing on the ground in pain. Not to mention the people who would scale to those characters doesn't make much sense. If starkiller is Large Star level then so would Rahm Kota. If Rham Kota had power rivaling the Emperor then Vader wouldn't have sent his apprentice who's never fought a jedi after him..
I still support this. Vader is canonically Dooku's superior, who in every single source I have seen has been portrayed as Windu's superior or equal, more often the former, even as a weaker jedi than the sith he would become.