The absolute worst: Mariah Carey (seems her name comes up a lot under worst remakes) and Luther Vandross: Endless Love
Yes, that "All Cried Out" remake was horrible, as was Slow Jam.
Whomever remade "Ribbon in the Sky" (K-Ci, JoJo?)?? It was bad.
I'm sure there are other really bad ones that will come to mind.