If you want the truly worst songs ever, you probably have to go back to the 70's. Consider yourself lucky if you don't know any of these.
Muskrat Love - Captain and Tennille
Seasons in the Sun - Terry Jacks
Wildfire - Michael Murphey
Candy Man - Sammy Davis Jr.
We Built This City - Jefferson Starship
Wind Beneath My Wings - Bette Midler
Ebony and Ivory - Paul McCartney and Stevie Wonder
also, anything by Celine Dion
and I agree with Hlao about Amy Winehouse, yuck...