You have to consider how young they were when they started. Hamill and Fisher were essentially just unknown teens. Both get much better as the movies progress, I think due to growing up and building their experience. Harrison Ford was not a frontrunner for the Solo role. He was there to help others screentest, but he nailed the role in interacting with the others. Portman's role, esp in Ep3, I don't think is her fault. It is due to the lousy writing that Lucas gives her. She has all these flat lines and ackward love scene moments. She has a decent amount of action in both Ep1 and Ep2. But Ep3 just was basically focused on Anakin, the subplot with Padme's senatorial role having been dropped. Hayden though had a lot more blame, given that he was to become Darth Vader. More important lines, more activity in the movie. I think he fell flat on many lines, just didn't deliver them with the emotion that his scenes needed. Obi-Wan and Palpatine both delivered their lines quite well. I felt they really brought the Star Wars mystique into Ep3.