My personal favorite dialogue moments are first from Fallout 1...
The Vault Dweller approaches a man wearing armor and equipped with a large machine gun and a scowl. There is a name tag on his chest that says, "Chris".
Chris (menacingly): Wadda you want?
Vault Dweller: A burger.
... and one from Fallout 2...
The Chosen One is in conversation with a cranky drugs/supplies dealer named Renesco "The Rocketman" in New Reno, during conversation Renesco says the following, opening up the line after that to the player.
Renesco: I sell drugs to the addicts around here, which is how I earned the idiotic appellation, "The Rocketman".
Chosen One: Rocket Man, eh? Like the line in the Elton John song, "Rocket Man, and I think it's gonna be a long, long time. Burnin' at the speed of mmmnfcmm"?
Renseco: (Looks at you like he is undressing you with his mind, then tearing you limb from limb)
Do you have any other lines of which you are particularly fond? Please tell me!