I wouldn't want to take that on, because I have other lists keeping me occupied right now. If someone else did, I could provide some input.
I take it "(Sittin' On) The Dock of the Bay" would be #1. "Me and Bobby McGee" and Biggie's "Hypnotize" would also be high. I believe Johnny Ace's "Pledging My Love" is posthumous, and if so it would be very high.
Maybe posthumous songs would be better than posthumous singles. I think such a list would make more sense if to be eligible, the song must not have been released in any commercial form prior to the artist's death, meaning, for example, that "A Change Is Gonna Come" by Sam Cooke wouldn't be eligible.
I can start working on this list since I don't really intend it to be on main site and I want to do it just for fun, but if somebody more knowledgeable would like to take it, I will step out. And posthumous songs is probably better idea, you are right, Brian.