Anthony Daniels made 3PO a brilliant character, and provided the "over acting" comedy far better than any (*cough* Jarjar *cough*) since. "Don't risk yourself on my account, I'm done for.", "R2!? R2! Sir if any of my spares or parts will help him I'll gladly donate them!", "I'm sure master Luke will be alright R2. He's quite cleaver you know... for a human being.", "But Sir! Nobody gets upset about beating a droid!" "That's b'coz a doid don't pull peoples arms outta their sockets when they loose. Wookies've been known to do that." "I see your point Sir. I suggest a new strategy R2; Let the Wookie win."
Even Mark Hamills' over acting is legendary "No. That's not true. That's impossible!!!".
Of course, Harrison Ford can do no wrong. While Han and Indianna Jones are quite similar characters in action, Han isn't a professor, and nobody but Harrison Ford could ever be Han Solo. Likewise, I've seen a number of people trying to play Laia for side-story dramatisations, and nobody captures the mix of smart, strong, regal and vicious that Carrie Fischer managed.
The problems with the more recent trilogy, (1, 2 and 3) for me, basically fall into the age of little Anney. He should, at the very least, have been a young teen in that movie. If he's going to be romantically attractive to Pademay, he needs to be at least 14 when she meets him, otherwise she's never going to see him as anything more than a little brother. Add to that the fact that she'd already been a planetary ruler, and the only way he could possibly romance her would be to use Force Persuasion, and the Jedi council would recognise such an misuse of the Force as Dark Side power from the beginning.
Jarjar is a nightmare character and should never have been included, at least not in that form. However, if I simply image Anakin being more 15 than 11 during Episode 1, the rest of the movies fall into place much better.
Jake Lloyds' portrail of Anakin was fine, if they'd waited for him to be old enough for the part to work, and if there had been 4 or 5 years between the making of each of episodes 1, 2 and 3 as there had been between 4, 5 and 6 then he could probably have carried the part to it's conclusion. He'd still be a little young to perform his transformation (25 ish, when he should really be 28 - 33) but a little make-up and careful acting would have covered it.
Hayden Christensen seems to be incapable of expressing any emotion, and would be far more suited to playing a Star Trek Vulcan than the most emotionally tumultuous Jedi in a history which spans 10'000 years. (seriously, Revan and Malik have nothing on this guy, and even through Vaders Manipulation Starkiller is just as much as spoiled and frightened child as Maris Brood... all awesome characters in their own right)
Ian McDiarmid replaced the Palpatine of Episode V (Empire Strikes Back) for Jedi, and returned for episodes 1, 2 and 3; His acting was always outstanding, throughout. In fact, it was he who kept me going most through the early trilogy. The character of Palpatine is an interesting one to follow as well, and I wish I knew more of his journey before joining the senate.
I'm a big fan of Liam Neeson back from his role in Krull, and I actually think it's a shame that he was too old, by the time Episode 1 was being shot, to have played Obi-Wan. Never the less, Qui-Gon Jinn is still an amazing character, and it's a great shame that Episode 1 is the ending of his story. The hardships and romance of his life before that film is something else I'd very much like to see revisited with Liam in that role.
I still find it impossible to see Ewan McGregors' Obi-Wan Kanobi ever becoming that of Sir Alac Guiness. Now the shoes of Sir Alac Guiness are some serious shoes to fill, but even so, it's a film he thought was complete rubbish and a role he put very little effort into. I was very sadden to see Ewan do such a poor job of such a pivotal role in the first 3 episodes. Of course, it's probably very hard to act out a close emotional friendship with an emotionless plank, and by order 66 we would expect Commander Codys' betrayal to be harder for Obi-Wan to come to terms with than Anakins.
If I could have chosen anyone I liked for that role, I'd hope Jude Law would be the kind of Obi-Wan I'd have wanted to see... but maybe he didn't want the part? IDK.
I'm also unsure who I'd want to be Anakin, just not Hayden Christensen. Maybe Benedict Cumberbatch or Zachary Quinto? Really, it should have been someone fair haired, or the genetics don't really work, but acting ability is probably more important. It is a fantasy universe after all.