I find my one-liners and witty retorts are found as purity in the crucible of multiplayer, where all the crap floats to the top and you have to fight through it to be heard.
One time I was playing against some French in Worms 2, and they were badmouthing my country, etc. which I didn't particularly care about, so I said to them "Je deteste vous, je deteste votre payes, je deteste votre norriture, je deteste votre langue et je deteste votre visage."
Which I found hilarious at the time.
Later on, they said "The queen suck black dick", to which I responded "Which means she gets more action than you ever have."
But my real one-liners come from FPS's. Memorable ones include:
*When playing Wolfenstein: Enemy Territory, I was running out in the initial rush, only to my horror to see that an enemy Field Ops had thrown out an airstrike beacon right in front of me. Instinctively, I kicked out as the Field Ops opened the door of a building to take cover, knocking the beacon into the building with him, causing him to be splattered by his own airstrike. At this point, I quipped "I'm one foot ahead of you."
*When playing Call of Duty 4, I noticed somebody on the other team had the name Wolfenstein. Getting a headshot on him with my R700, I then quipped "Looks like you're in Enemy Territory now."
*Others include my Left 4 Dead ones, like when I killed a Smoker after his attempt to grab me failed, and the guy just lunged at me in the attempt of doing some damage. I pushed him back into the flames and said "No smoke without fire."
*Or whenever I kill a Boomer before it can vomit, and it doesn't spatter on anybody, I say "Looks like somebody detonated prematurely."