Depends on the genre.
Easy Listening - The Sounds of Silence (Simon&Garfunkel/Disturbed)
Folk - All Along The Watchtower (Bob Dylan/Hendrix)
Metal - Into The Void (Black Sabbath/Exhorder)
Instrumental - Little Wing (Hendrix/SRV - Live at El Mocambo)
What is your favorite cover?
There are many great covers, such as Mr. Tambourine Man,by the Byrds. I'm curious about everyone's favorite.