The Liquid Plumr commercial, "Double Impact". The ad features two of the greatest lines I've heard in some time, "I’m here to snake your drain," and "I’m here to flush your pipe". The most surprising thing about this spot is that they didn't pull this off sooner.
Credits:
Title: Liquid Plumr "Double Impact"
Agency: DDB San Francisco
Director: Clay Weiner
Production Company: Biscuit Filmworks