Unfortunately, the best performance I've honestly seen on Idol is Chris Connelly's "Billie Jean", changed to acoustic and electric guitar and time changes to 3/4 time, like a waltz. David Cook chose an obscure cover instead of playing it straight and safe and yes, a rip-off of Connelly's, but Connelly had to sign off on the use of the song in that performance so call him a sell-out to the Idol machine as well.

I'm glad someone already pointed out Johnny Cash's Hurt... Other great covers include A Perfect Circle's cover of Depeche Mode's People are People (A Perfect Circle's album Emotive is all great covers!) and more obscurely His Name is Alive's cover of Man on a Silver Mountain sung with just a treated guitar and 2 high pitched almost falsetto female vocals.

I had fun compiling some of my favorite covers:

Hallelujah (Leonard Cohen) covered by Jeff Buckley

Bizarre Love Triangle (New Order) covered by Frente

Please Please Please Let Me (The Smiths) covered by Muse

All Along the Watchtower (Bob Dylan) covered by Hendrix

Love Will Tear Us Apart (Joy Division) covered by Fall Out Boy

Dear Prudence (The Beatles) covered by Siouxsie &; the Banshees

Just Like Heaven (The Cure) covered by Dinosaur jr

Superstar (The Carpenters) covered by Sonic Youth

Running up that Hill (Kate Bush) covered by Placebo

The Man Who Sold the World (David Bowie) covered by Nirvana

Wonderwall (Oasis) covered by Ryan Adams

Blue Monday (New Order) covered by Orgy

Mad World (Tears for Fears) covered by Michael Andrews

Aint no Sunshine (Bill Withers) covered by Eva Cassidy

Across the Universe (The Beatles) covered by Rufus Wainwright (Fiona Apple too)

Sea of Love (Phil Philips) covered by Iggy Pop (good Cat Powers version too)

Purple Haze (Hendrix) covered by The Cure

St. James Infirmary Blues covered by The White Stripes

A Change is Gonna Come (Sam Cooke) covered by Cold War Kids