Both Portman and Jackson's performances/ delivery actually made the dialogue seem worse. That is quite an accomplishment!
For the most part MacGregor, Neeson, and McDiarmid made it somewhat better or had no effect. That's not saying much of course. McDiarmid was the only one who seems to have understood the theatrical style that Lucas was (stupidly) going for and was also able to tailor his performance accordingly. But even he, at times, seems to have been playing a spoof of Palpatine.
Christensen is all over the board. Sometimes he butchers the dialogue, sometimes he has no effect, sometimes he made it better. I love the look on Hayden's face (in RLM's review of ROTS) when Lucas informs him that he changed his dialogue over the weekend!