So many great ones... the one that comes to my mind is from Dead Rising. It's not really much of a giveaway, but I'm throwing the spoiler tag up just in case:
One of the bosses is a crazed grocery store manager. After you've beaten him, he lies in a pool of blood blabbering on about who's going to run cash registers, clean, and take care of the store once he's gone. After he finishes it looks like he's dead, when suddenly he jerks his head up and screams "CLEANUP ON AISLE SIX!!!". I don't think I've ever laughed so hard at a game in my life.