Ahmed Best, because he's playing a character which is (I assume) supposed to be endearing, and ends up making him damned annoying.
Hayden Christensen.... do I even need a reason?
As much as it pains me.... Natalie Portman. Congratulations, George Lucas. Your shitty scripts for the PT made even Natalie Portman look like a bad actor. Seriously, in Episodes II & III in particular, she has this look like she can't believe the mediocrity of the dialogue she's reciting. And I'm right there with her.