Sam Jackson was Sam Jackson in a robe, which is fine I suppose, but it's out of place in a Star Wars movie.
Well... he was an awfully subdued Sam Jackson. The part sure didn't make much use of his talents. Also, Windu came off arrogant and was often annoying. And Sam Jackson doesn't fit prequel Jedi and they don't fit him. Jackson is a really good actor, but a lot of the time in the prequels he was just annoying.
Ian McDiarmid was awesome, both as the fairly subtle (for Star Wars anyway) Palpatine and the cackling Sidious (man did he look like he was having fun). The best of the lot.
Once ROTS got to the point when Windu came to arrest Palpatine, Ian's performance turned to shit and never recovered. He delivered some excruciating "acting" in the later part of the movie.
Ewan McGregor was commendable, his acting wasn't always terrific, and sometimes it got a bit hammy even, but he clearly put a lot of effort into "being" a young Obi-Wan, so I can't complain too much. He is capable of better, though.
Ewan was ok in TPM, but in the later two films his performances were just bland and artificial and sometimes annoying. A key thing about the later two prequels is that all three main characters came across false and were unrelatable. Kenobi in the later two films, particularly the second, came off like a pretentious poser.
Liam Neeson was good, as he usually always is, shame he had no part in the second two, he would have helped them a lot.
Neeson's one the best actors around. In TPM he wasn't at his best, but he delivered a great performance anyway. Easily the best thing in the prequels.
Christopher Lee was given nothing to do, but he did it with dignity as always, a wasted oppurtunity though.
I found Dooku annoying. He came off like such a pompous effete poser. And his duellist pretentions made me want to throw something at him. The EU builds up his duellist thing farther and is quite worshipful about him. They even have Yoda say at some point that he was the best and wisest of the Jedi or something like that. The EU likes to build up characters most worshipfully -Mace Windu the bestest lightsaber duellist who's so cool he's got a fighting style only he can master without going dark and which plays with the dark side without going into it (he also he also a special sort of prophetic shatterpoints power and was on the council at age 28, because he's, like, the coolest), Count Dooku, also the bestest lightsaber duellist, who's just so wise even though he's evil (oh how tragic is it that this great Jedi went dark), Jango Fett who can kill loads of Jedi with his bare hands because he's the coolest Mandalorian warrior ever before lil Bubba (and we all know Mandalorians are sooo cool), etc, etc.
Characters in the prequels who are more annoying than Jar Jar: Anakin, Kenobi, Padme, Yoda, Windu, Dooku, Jango, Boba, Nute Gunray and his pals, Later-ROTS Palpatine, various silly droid troopers of various types, various podracers.