You guys have it all wrong
First off, Anakin was a billion times superior to Vader. Why? Because he wasn't a crippled bastard dragging an oxygen tank around everywhere as Anakin. Anakin, if he hadn't been owned by the lava, would have become the most powerful Dark Jedi of all time. Period. He would have been at the absolute top of his game.
Anakin could use a lightsaber like nobody else. All that went to shit when he became the robotic dead. His mastery in the force continued (Though we don't see it in the movies because of the special effects; Anakin appears more powerful with the Force than Vader does for precisely that reason, in the 70's/80's the Force was limited to very simple stuff, it became unlimited with the advances in special effects, however), but his lightsaber skills only deteriorated.
In fact, during his post-Order 66 Purge of the Jedi, Vader nearly got himself killed. He was clumsy and unaccustomed to working with his new body, and almost got owned by a Padawan.
THIS IS CANON PEOPLE.
Obviously in the next twenty-odd years he made advances, but he never reached the same heights he could have pre-suit.
As for Grievous:
He was a fag. The character didn't really have any originality to it, half machine half human? Sounds like a certain Dark Lord of the Sith I know.
Sure, the four hands thing was cool, and the possibility that he could have mastered lightsaber technique beyond anything capable by a human, but his whole lack of the Force shot that entire thing down.
He became weak next to a Jedi, all they had to do was use the Force against him, as Obi-Wan demonstrated.
He was an amateur. Dooku was a far more interesting bad guy, and even though he was weak and pathetic too, he at least had mastery of the Force.
So, both of them are certainly not the best characters in the series, to do with power anyway, but both are pretty cool.