I would say some of the Portal quotes since they make me laugh so hard, but I think I will go for some classics:
Minsc (Baldur's Gate 2) when he is displeased with your decisions: "This behaviour must not continue. Feel the burning stare of my hamster and change your ways."
Link to the wav file: http://www.minscandboo.com/quotes2/This_behaviour_must_not_continue.WAV
Oh and Hk-47 from KotOR... here are some of my favorite quotes from him:
1) "Observation: I am a droid, master, with programming. Even if I did not enjoy killing, I would have no choice. Thankfully, I enjoy it very much."
2) "Definition: 'Love' is making a shot to the knees of a target 120 kilometers away using an Aratech sniper rifle with a tri-light scope... Love is knowing your target, putting them in your targeting reticule, and together, achieving a singular purpose against statistically long odds."
3)
HK-47: "Commentary: How would you like to be the wholly-owned servant to an organic meatbag? It's demeaning! If, uh, you weren't one yourself, I mean..."
Revan: "Demeaning, is it?"
HK-47: "Qualification: Err... perhaps I did not mean it quite like that, master. I mean... while an artificial life-form is superior..."
Revan: "You're not making it any better."
HK-47: "Commentary: I mean... nice human, goo-oood human..."