I'd like to make an addendum to this thread. Here is another quote declaring Sidious to be the strongest Sith Lord of all:
Finally - the Emperor - who should be fighting to save the Republic - is revealed as the most powerful and menacing Sith of all!
--The Sith (Revenge of the Sith Collection)
I'd also like to add another section entirely:
Palpatine is the living embodiment of the dark side of the Force:
Palpatine's fury buffeted Plagueis. Blossoms growing along the sides of the pathway folded in on themselves, and their pollinators began to buzz in agitation. FourDee reacted, as well, wobbling on its feet, as if in the grip of a powerful electromagnet. Had this human truly been born of flesh-and-blood parents? Plagueis asked himself. When, in fact, he seemed sprung from nature itself. Was the Force so strong in him that it had concealed itself?
--Darth Plagueis
A welter of voices, near and far, present and from eons past, drowned his thoughts. Raised in praise, the voices proclaimed his reign and cheered the inauguration of a new order. Yellow eyes lifted to the night sky, he saw the trembling stars flare, and in the depth of his being he felt the power of the dark side anoint him.
--Darth Plagueis
In his mouth, Sidious tasted the tang of blood; in his chest, he felt the monster rising, emerging from shadowy depths and contorting his aspect into something fearsome just short of revealing itself to the world.The dark side had made him its property, and now he made the dark side his.
Breathless, not from exertion but from the sudden inspiration of power, he let go of the sill and allowed the monster to writhe through his body like an unbroken beast of range or prairie.
Had the Force ever been so strong in anyone?
--Darth Plagueis
The feeling had begun as a faint stirring in the Force, like the tiniest ripple of something moving slowly through deep water, far away but drawing steadily closer. It intensified, until it felt like the Force itself was roiling, heaving like the sea in the grip of an enormous storm.“I sense a presence,” Maul warned Savage. “A presence I haven’t felt since...”
And then Maul knew.
“Master,” he said, leaning forward on the throne.
The commandos guarding the royal chamber reached for their throats. As Maul watched, an unseen forced lifted them high in the air, then slammed them to the floor, where they lay motionless in their red-and-black armor. The doors opened, then closed behind a figure in dark robes. A deep cowl hid most of the face, leaving only a pale chin and a downturned mouth visible. To most eyes the man in those simple robes of rough cloth was unremarkable, just another being making his way in the universe. But to those who could feel the Force he was anything but ordinary. To them, he was a dark sun blazing with power that was simultaneously hypnotizing and terrifying to behold.
Darth Sidious, the reigning Dark Lord of the Sith, had come to Mandalore.
--Darth Maul: Shadow Conspiracy
And then there was Palpatine, of course: he was beyond power. He showed nothing of what might be within. Though seen with the eyes of the dark side itself, Palpatine was an event horizon. Beneath his entirely ordinary surface was absolute, perfect nothingness. Darkness beyond darkness.A black hole of the Force.
--Revenge of the Sith novelization
Depowered lampdisks were rings of ghostly gray floating in the gloom. The shimmering jewelscape of Coruscant haloed the knife-edged shadow of the chair.This was the office of the Chancellor.
Within the chair's shadow sat another shadow: deeper, darker, formless and impenetrable, an abyssal umbra so profound that it drained light from the room around it.
And from the city. And the planet.
And the galaxy.
--Revenge of the Sith
When those blades met, it was more than Yoda against Palpatine, more the millennia of Sith against the legions of Jedi; this was the expression of the fundamental conflict of the universe itself.Light against dark.
Winner take all.
--Revenge of the Sith
The key to Luke's turning is the moment when he and Leia realize the Emperor is no longer defined by his physical form, but has become a chaotic nexus of dark energies that swell and burst open the fabric of space, tearing apart everything in the vicinity, human and machine.
--Dark Empire endnotes
The 10 years that followed were tumultuous, but nothing rivaled Hethir's shock when summoned to the planet Byss to face a miraculously reborn Emperor Palpatine. Hethir re-pledged his loyalty immediately, coming to understand Palpatine not as a man but as a vessel for the darkness of the universe, an avatar of the dark side itself.
--Aliens in the Empire, Part II