From Mass Effect, the one-liner you didn't get to say because some of the dialogue (okay, most) is cheesy as hell. The short mission where you choose to either give the diplomat guy the depressant, the stimulant, or just fuck him and never go back. Give him the depressant and he asks "Wha...what did you jut give me?" And there it is, one of the best lines in the game, ready for your saying.
"A lesson." After which you presumably turn around and coldly stroll out.
But when you pick it, your character grabs the guy by the throat, pushes him up against a wall, and basically shouts "DON'T DO DRUGS THEY'RE BAD FOR YOU I HOPE YOU LEARNED SOMETHING".
Or, anything Wrex said.