Any Tim Schafer game has dialogue that'd put most sitcoms to shame:
From Day of the tentacle:
Dr Fred: I think I've made myself totally clear, Step one: Find Plans, Step two: Save World, Step three: Get out of my house! Let's get cracking!
And, possibly the coolest game ever made, Grim Fandango:
Mannny: Glottis... Glottis... is that a German name?
Glottis: Oh, no. My roots lie not in any Earthly nation's soil. I am an elemental spirit summoned up from the Land of the Dead itself and given one purpose, one skill, one desire... TO DRIVE. Or, to change oil or adjust timing belts if no driving jobs are open.
Manny: As a rule, I never touch anything more sophisticated and delicate than myself.
And, yet again, Psychonauts:
After being presented with a "communications device"
Raz:Is that... bacon?