Gladys, Otis, Daryl and Stevie: great additions. Daryl is such an underrated vocalist. And as much as I love XTC (and I do), I wouldn't have thought to include Andy Partridge in this thread.
I was trying to think of an XTC song also, but the closest I could get was Colin's vocals on Making Plans for Nigel. Their genius was song writing and arrangements imho.
I'll try to stop adding more but I thought of this one last night
Ian Curtis (Joy Division) - Transmission (The BBC Sessions)