Hurt (NIN) by Johnny Cash 🙂
All Along the Watchtower (Bob Dylan) by Jimi Hendrix - as previously mentioned, this is the definitive version of the song.
Hallelujah (Leonard Cohen) by Jeff Buckley is better than the original, but I like the Rufus Wainwright version the best.
Turn the Page (Bob Seger) by Metallica - actually I'm kinda even on this one, they are equally good imo.
Crossroads (Robert Johnson) by Cream.
Cocaine (J.J. Cale) by Eric Clapton.
Hush (Joe South) by Deep Purple.
Mad World (Tears For Fears) by Gary Jules - shoulda seen that one coming.
Respect (Otis Redding) by Aretha Franklin - this is the definitive version of the song.
Statesboro Blues (Blind Willie McTell) by the Allman Brothers Band.
Summertime Blues (Eddie Cochran) - Alan Jackson.