Padme
She was nothing. I know nothing more about Luke and Leia's mother now then I did 6 years ago. She simply exists. Her profession of love for Anakin and her constant babbling about democracy does not make her a character. She is exactly the same person when she dies that she was when she was introduced. Every single scene she is in makes me cringe. Her main function was to make the audience to realize that Lucas can not write a love story.
Its only fair that I admit that a character's importance in the plot was my main criteria. She barely edges out Anakin because there are scenes where I saw Anakin change or do something cool. There was none of this for Padme Amidala, she sucks.