Uhura. Although she's fondly remembered as part of the main cast from TOS, she rarely had anything of significance to do. She had some good moments early in the series, but her part just kept dwindling and dwindling as things went on.
I disagree. I think she got more good lines in the second season than the third. And no part was less substantive than her broken record of "hailing frequencies open" in The Corbomite Maneuver.