I just watched the Star Wars Episode 1 Rifftrax for the first time and holy crap is the acting bad. I didn't remember it being this bad. When I saw it in the theaters I didn't notice as much because all those long talky scenes put me to sleep and then the explosions woke me up and I only saw the action.
Anyway, I vote for Natalie Portman, although she was thoroughly upstaged by Hayden Christensen in the other two Star Wars prequels, who invented new ways to suck at acting. Seriously, what the heck is with the acting in these movies? Samuel L. Jackson is not a bad actor, it's like they were directed to give wooden and robotic performances. Why can't they put good acting in this CGI Monster-filled enviroment. Troll 2 has better acting than these movies! Compare the kid in Troll 2 to the kid in Episode 1.