Ugly opinion is that Rod Stewart's DOWNTOWN TRAIN is better than the original. Not that it's a great song anyway.
Warning Ansley Dunbar Retaliation http://www.youtube.com/watch?v=8U4GixWwk_k&feature=related & by Sabbath http://www.youtube.com/watch?v=ZcxEp9I5k1g&feature=related Sabs is better.
ZZ Top - Viva Las Vegas http://www.youtube.com/watch?v=KLUl8xkKSTk - It's not completely better than the original, more like an homage. Guns n Roses - Sympathy for the Devil http://www.youtube.com/watch?v=xmHZLruBSMs&feature=related - I'm sure a lot of you have this soundtrack in your collection!
I was trying to think of a Depeche Mode cover that improved the Depeche version. There aren't any. Gravity Kills do a decent Personal Jesus, Rammstein do a cool Stripped, Deftones do a meaty To Have and to Hold and In Flames heavily brutalise Everything Counts, but no one beats the Mode it seems. Marilyn Manson and Lacuna Coil, of course, ruined their songs.
The Jam's version of "David Watts" was a definite improvement on the original version (by The Kinks'). GM
I think William Shatner's version of Pulp's Common People is better than the original, Shatner's vocal inflections really make this work, he gives it this great snark, and Joe Jackson is great. http://www.youtube.com/watch?v=KXWEM4gZhg4 (good sound - set to Star Trek TAS scenes) http://www.youtube.com/watch?v=AzlwKNbwxkw (best sound - some dude lip-syncing) And I love Nina Gordons' cover of NWA Straight Outta Compton even though it's so wrong (or maybe right...) http://www.youtube.com/watch?v=K6D5xpCgETk I never knew that was a cover but I'm digging Mac's version right now....
Green Manalishi with the three pronged crown - Judas Priest's version is FAR superior to the original by Fleetwood Mac
When did they add the 3rd prong? Ok, I'm being facetious, don't listen to me, listen to Diamanda Galas. http://www.youtube.com/watch?v=b9ntyYWgX7c&feature=related
Not quite rock (Ben Folds is rock, just piano/geek rock, but...) Ben Folds' version of Dr. Dre's "Bitches Ain't Shit". Now that is how you do a cover. http://www.youtube.com/watch?v=ic7KH1PpbMY - The original by Dr. Dre... meh. http://www.youtube.com/watch?v=dSJxvi767kQ&feature=related - Ben Folds cover (ignore the video, some fan video)