Damn... I can't pare it down to one!
Okay, Favorite:
Baldur's Gate:
"(to Portalbendarwinden): Ok, I've just about had my FILL of riddle asking, quest assigning, insult throwing, pun hurling, hostage taking, iron mongering, smart arsed fools, freaks, and felons that continually test my will, mettle, strength, intelligence, and most of all, patience! If you've got a straight answer ANYWHERE in that bent little head of yours, I want to hear it pretty damn quick or I'm going to take a large blunt object roughly the size of Elminster AND his hat, and stuff it lengthwise into a crevice of your being so seldom seen that even the denizens of the nine hells themselves wouldn't touch it with a twenty-foot rusty halberd! Have I MADE myself perfectly CLEAR?!"
-The Protagonist
Honorable Mentions:
Diablo II:
Hratli
"I see you're going to Hell before me. Be sure to put in a good word."
Fallout:
President John Eden
"Yes, I do. Even killing me won't help you. There's nothing you can do to stop the release."
Chosen One
"I disagree, Mr. President. I think killing you will help. I'll feel a lot better."
Fallout 2:
Chosen One
"You are one sick puppy. Time to put you down."
The Witcher
Boss
"Draw your Sword!"
Geralt
"I was afraid you'd suggest chess."