Yeah, I might have agreed with McDiarmid's preformance being good as far as TPM goes, and even a good degree in AOTC, but from start to finish I thought he was one of the absolute worst parts of ROTS. Straight from the boot he was way over the top, I was cringing all throught the mock throne room scene the first time I saw it. That scene had so much potential, but then again by that point between the imaginative buzz droids and the CG clone pilots and Oddball, the movie had already blantantly admited that it wasn't going to try too hard to impress.

Just for fun, here is how I would rate them,

McDiarmid (Palpatine) B
McDiarmid (The Emperor) F- (and that is only because I can't give him anything lower than an F. And yes, I know there is no such thing as an F minus)
McGregor B- (Excellent from start to finish in TPM, wasn't as great in AOTC or ROTS, way too any dumb lines which isn't his fault, but he had moments. Felt kind of like he had given up on the series and was just fullfilling his contract. He was the only good thing about the end of ROTS though, that is one moment when he really shines.)
Christopher Lee A (Still proving himself to be the perfect villian. Man, what I wouldn't give to see him play in a good SW movie, it would be a dream.
Liam Neeson A (The poor guy carried a good deal of TMP by himself, I would hate to see what the movie would be like without him. Plus the poor guy had to put up with having the most interaction with Jar Jar, the worlds first 100% CG character. The fact that he, being a serious actor, had to humiliate himself so much to do this film should be enough to earn him an A right there.)
Pernilia August C- (Good in TPM, horrible in AOTC, but then again she wasn't given much to work with)
Hayden Christenson D-
Natalie Portman F (just because she has proven elsewhere that she can do better)
Jake Lloyd --- E for effort.