The best cover tunes are the ones that many Jam Bands play during live shows, but never release on an album
Phish does a ton of covers and since they allow the recording of there concerts all those covers are well documented
Same goes for other Jam Bands like the Grateful Dead, Moe, Galactic, Karl Densons Tiny Universe, Government Mule, etc,...
Many of these bands play some really nice long versions of kick ass tunes
but obviously its a matter of taste cause most of the tunes listed here kind of make me naseaus (sp?)
I will say this "Knocking on Heavens Door" by Guns and Roses is by far the BIGGEST butcher job EVER !!!!! I laugh every time I hear it