I don't know about THE worst song I've ever heard - too of C&W candidates to choose from for starters - but when it comes to the most unintentionally funny, there can only be one.

Ken Dodd's song 'Happiness'.

A bad song made worse because it comes out as:-

A penis, A penis, The greatest gift that I possess. I thank the lord that I've been blessed With more than my share of a penis.

It cracks me up everytime. I have to leave the room.

Brian