Master Crimzon
Baby Killer
Which is exactly the point: Sidious' very forte is not his technical prowess over the lightsaber (which is highly impressive, regardless), but rather his ability to use the force in order to enhance his speed to a frightening extent.
In fact, that speed was far more than Mace Windu had the ability to handle before submerging into Vaapad; he was forced back with the sheer ferocity and speed behind Sidious' style. He also did not have the speed to react to Sidious' slaughter of two Jedi Masters with one blow each, only being capable of engaging him once Kit Fiisto blocked his attack. Note; this is the same Windu that is noted to have a spectacularly powerful physique and force abilities outside of his usage of Vaapad, substantiated by feats. This is also the Windu that, without the aid of Vaapad, was capable of forcing Grievous on the defensive within their short fight, although of course it can be argued that Grievous wasn't able to use his full might in that fight.
Similarly, Grievous' style played directly into Obi-Wan's hands: it lacked any precision and finesse behind it, the traits that have been shown to be Obi-Wan's bane (his battles against Count Dooku). And even then, Obi-Wan was eventually overpowered at 20 strikes per second. It was to his fortune that Grievous did not have the sufficient skill to parry his attacks. Sidious? He's hardly of the same caliber. On the other hand, Anakin, using the force of the dark side, was able to force Obi-Wan back during their duel and lead him to a point of desperation and physical exhaustion (this can be seen from his expressions in the movie, his entire body language, whatever). Obi-Wan knew Anakin's every movie, while he would be completely unfamiliar with Sidious' erratic and unpredictable form of combat; not to mention that Sidious, having far superior control over the force than Anakin, is capable of harnessing speeds that far eclipse Anakin's own.
Moving with speeds that transcend those of opponents that heavily challenged Obi-Wan = definitive win. It's speed that is superior to Anakin's, Mace's, whatever, both Jedi said by George Lucas and Nick Gillard to be level 9's- above Obi-Wan's level 8 by a considerable margin.
Another argument is thus: when Dooku battled Yoda, Yoda kicked his ass with his speed and ferocity. The count could not handle it. On the other hand, Dooku was more skilled than Grievous and clearly had the ability to defeat him in lightsaber combat. Even if Grievous did not go all-out in these sparring matches, it cannot be argued that Dooku was, ultimately, the superior swordsman. However, Sidious battled Yoda on completely even grounds- disarming speculation or whatnot, it was evident that they were well matched in speed and a closer look at the duel on the senate pods would reveal that Sidious definitely held the offensive.