Even if you ignore the statements that outright confirm his superiority over every other Sith in the mythos, simply take a look at what the man himself has done:
He blunted Jedi sensitivity on a galactic scale throughout the prequel trilogy; he was the individual responsible for the Jedi's inability to gaze into the future, confirmed by the official databank and RotS novelization.
He defeated three Jedi Masters who were universally regarded as "celebrated swordsmen" according to the Revenge of the Sith Visual Guide. The narrator of the RotS novelization regarded one of them (Agen Kolar) as one of the best ever along with Mace Windu. Obi-Wan Kenobi would personally count them all among the greatest swordsmen in galactic history. Keep in mind: the first two he killed with one swing of the lightsaber each and the third he butchered while simultaneously grappling with Mace Windu. He would also "force the mighty Mace Windu back" according to the official databank before being defeated by Windu's mastery of Vaapad and the shatterpoint charism.
According to The Essential Guide to Characters, his "mind fogging powers" manipulated millions of Coruscant citizens to forget the construction of the Super Star Destroyer Lusyanka.
His Force lightning was enough to disintegrate the flesh of three powerful Prophets of the Dark Side who were well versed in Sith alchemy which, according to the Dark Side Sourcebook, is reserved for high end Force users. The lightning was also sufficient to instantly destroy fifty stormtroopers who attacked him; Palpatine also controlled the bolts to work around the Royal Guardsmen who were embroiled in the melee. It was also sufficient enough to nearly overwhelm and kill Mace Windu who had the advantages of greater strength, mass, leverage, a lightsaber, and the metaphysical conduit of Vaapad.
Lastly, he was capable of summoning hyperspace wormholes that could not only "tear the surfaces off worlds" but also obliterated a fleet of starships equipped with shields capable of shrugging off gigatons of damage.
That doesn't even begin to cover most of his feats. Palpatine is, essentially, the Sith god.