ooo good one....ummm here is the best 5 covers that i can think of at the moment anyway, in no particular order
guns and roses - knocking on heavens door (original by bob dylan)
grinspoon - dont change (original by inxs)
jimi hendrix - all along the watch tower (original by bob dylan)
the ataris - boys of summer (original by don henley {i know some people will shoot me for that one, but i think the cover is a good song too})
ok this one is a bit weird
i was pretty sure that bob dylan sang the original (i have since found out that he has covered it too) but i couldnt remember if pink floyd covered it or they just played it live (i got the mp3 on the computer)... but anyway
pink floyd - house of the rising sun (not sure who did the original but here is the web site anyway
http://en2.wikipedia.org/wiki/The_Ho...the_Rising_Sun) i have also heard versions by the animals and bob dylan and they both kick ass aswell