Maul and Grievous kind of filled their roles as big scary bad guys that had to be fought and defeated. They were like scene bosses in Diablo that you had to beat to get to the act boss. I'll admit I was always disappointed by Grievous though after seeing him wreck shit in the Tartakovsky cartoon.
Mace kinda filled his role too as the bad cop to Yoda's good cop but he seemed pretty bland outside of that.
I would have liked to see more of the political maneuverings Dooku had to do to build up the CIS but the way the trilogy was made didn't really leave much time for that.
Jango was kinda pointless and should have just been cut, especially since he was just fanservice anyway.
Qui-Gon, Shmi, and Yoda all did what they were supposed to.
Jar Jar wasn't that bad outside of the stupidity of his antics in TPM. His stupidity being taken advantage of to issue the emergency powers decree was perfectly in character but was also kind of dumb.
I'll probably give the nod to Grievous because, while Maul was scary and imposing, Grievous was a bitch and had no buildup at all as to how or why he was so important. All he did all movie was look incompetent. Maybe that's how he was supposed to look since he was an expendable part of Sideous' plan anyway, but it still felt lame that he was such a target of the Jedi's war effort. He was like Jar Jar an droid form except we were supposed to take him seriously.
You're such a boring characterless entity. Try getting laid once in a while and maybe you'll have friends and find out what a "joke" is. - derrate