I voted for "The Beautiful Ones" ... but I'm like YesNo, if I really think about it, I'd probably have to admit that "When Doves Cry" would be the desert-island song. If I was forced to choose one song.
But having said that, "Beautiful Ones" is a pretty amazing showcase for Prince the vocalist. I like how it begins with the fruity, breathy falsetto ... and ends up on the opposite end of the spectrum, super growly (like Eddie Murphy's impersonation of Teddy Pendergrass).
I like almost all of this album, but it's got to be Baby I'm a Star. Seems like the sort of thing that could go on 10+ minutes in the right live setting.
Around the time Purple Rain came out, MTV was heavy-rotating an extended live-performance clip of Prince and the Revolution doing a medley of "I would Die 4 U" leading into "Baby I'm a Star". I think it was in that order. Sort of re-creating the end sequence of the movie, but the MTV thing was more of a real-deal live performance.
... in any case it was fucking great. I would get super-psyched whenever MTV showed this thing. It was that live clip that really illuminated Prince's greatness to me. I'd love to see it on YouTube or somewhere ... but unfortunately the Purple One is super-fanatical about banning any YouTube clips of his music. Like many geniuses, he's a fairly annoying person.