My favorite witty line was completely unrehearsed. I was playing lead guitar in a rock and roll band in a seedy, off-the-beaten-track bar in western Massachusetts. Two fellows who were drinking heavily and sitting close to the stage were apparently fond of a certain late-1960's band from LA, and kept requesting them by shouting "Doors! Doors!" between every song. After a while I finally stepped up to the microphone, pointed to the exit, and said "Door's over there". It got a laugh, and they actually took me up on it, so it worked out very well.

Pretty case-specific, but that's all I've got to offer.