It’s the summer of the cover songs, with albums by Mark Ronson, Bryan Ferry and even Poison recording other artists’ songs. Then there’s “Instant Karma,” a benefit album for Darfur, featuring John Lennon classics recorded by wide-ranging artists such as Black Eyed Peas, Big & Rich … and Avril Lavigne doing “Imagine” (the horror). Which got us thinking: What are the best covers of all time?