Demos are amazing. Wrong notes, tempo speeding up and down, warts, and all. If you fall in love with a demo version of a song, its hard to ever love a re-recorded version in an expensive studio as much.
But what is your favourite demo version you've discovered after already loving the released/album version.
For me its Radiohead's Motion Picture Soundtrack demo. Just Thom and a guitar. Extra verse to the Kid A version too. I would probably argue that the album version is actually better, but as the song is so special to me I love it anyway.