Nephthys
The Gr8est!!!!!!!!
The line has been pretty well drawn at maximum of affecting planets or causing planet-wide effects in the case of the most powerful non-entities, with your standard abilities. Rituals obviously can scale up to far larger than that or if you're using outside forces to amplify already potent powers. Sidious and Vitiates rituals and Sadows solar manipulations spring to mind.
Vitiate's ritual is ridiculously massive in scale, but he had prepped for it for a thousand years, was on an immensely powerful nexus and was being fueled by trillions of deaths from across the galaxy. He may also have been conducting it in some manner throughout all of the JK Act 3 as well, to be able to have harnessed the destruction on the various planets. It's still incredible, but it isn't completely implausible.
The idea that Sidious could cause infinitely greater destruction with a single standard attack is Arkham Asylum style crazy. To do stuff on levels greater than planetary clearly require rituals, time and immense aid.