Definitely the end of Portal 2.
“You know what my days used to be like? I just tested. Nobody murdered me, or put me in a potato, or fed me to birds. I had a pretty good life. And then you showed up. You dangerous mute lunatic.”
Scout from TF2 comes close.
“Say goodbye to your kneecaps, chucklehead.”
The Fallout ‘War never changes’ monologues are also really great.