I have to say I agree with Tpt. Remember that Obi-wan fought and defeated a Sith while still a Padawan, and from there did spend a lot of time learning saber technique, which you can clearly see. Yoda relied on the Force for saber fighting, you can see this by all the flipping around that he does; he wouldn't be able to do that without using the Force. Mace? Well, as was said, he did leave too many openings while fighting. He was a good Jedi, yes, this is obvious by the fact that he was the head of the Council. But best duelist? No, I would have to say that that was Obi-wan, who is one of the few Jedi who'd ever even had any actual duelling experience.
Anyway, Grevious. I didn't mind him so much. However, they totally screwed things up with the Clone Wars cartoon. Yeah, he was supposed to be good, but that was just ridiculous, and it made him look like crap in ROTS.
All the damn Grevious fanboys (hell, fanboys in general) really annoy me, though....