Not sure its the worst line, but it annoys the hell out of me: We were warned about Ferengi at the Academy.
Rather than be open-minded and willing to form his own opinions, Kim will just go along with the apparent stereotyping and institutionalised racism that goes on at Starfleet Academy--only to then totally forget everything he's been 'warned' about by being taken in by Quark's con.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)