I'm partial to:
GLADOS: "Well, this is the part where he kills us."
WHEATLEY: "Hello! This is the part where I kill you!"
CHAPTER 9: THE PART WHERE HE KILLS YOU
"You know what I hope is in space? Fire. I hope you go to space and get set on fire."
"What's your favorite thing about space? [sub]Mine is space.[/sub]"
"[deep voice]I'm proud of you, son. [normal voice]Dad? Are you Space? [deep voice]Yes. now we are a family again.
"Ahhh! I'm gonna meet the sun! OH NO! What do I say? Hi! Hi Sun? OooOOh boy!"
"For those of you who volunteered to have mantis DNA injected in you: I have good news and bad news. The bad news is we're postoning those tests indefinitely. The good news is, we have an even better test: Fighting an army of mantis men! Grab a rifle and follow the yellow line. You'll now when the test starts."
"Now, I know some of you may have questions. Like, 'what exaxtly do these tests entail? What was in that phonebook of a contract I signed?' Let me answer that question with an even better question. Who wants sixty bucks?"
"Now, you may think that you don't have any tumors. Well, don't worry. If sat on our folding chairs without a pair of lead underpants, we took care of that you."