Internally Deformed
Farseer of the Paranormal Abysm
I'm suggesting one potential method of assessing how improvable a song is (it's difficult to deny that some songs are more improvable than others - take Unleashed's first two albums; some really cool riffs throughout, but easily improved by more lead work). Bear in mind that though popularity of an artist among the general public is a pretty poor measure of quality, a song/cover's popularity among an artist's fanbase - who we already know understand the music, and "get" what's good about it - despite being composed of multiple "subjective" opinions, probably brings us much closer to an objective measure. This doesn't hold true in cases where a band radically alters their style, but you could say that the original fanbase sample no longer applies in that situation.