The Dirty Half-Dozen.

The cover song is a powerful musical institution, capable of producing tremendous good…or unspeakable evil.

6. Fall Out Boy & John Mayer - "Beat It"
When John Mayer and Fall Out Boy teamed up in 2008 to cover Michael Jackson's "Beat It", it looked intriguing on paper: a Grammy-winning singer-songwriter and a Grammy-nominated pop-punk band covering a classic track from the King of Pop.

But the problems begin with Fall Out Boy vocalist Patrick Stump, who sounds on the verge of hyperventilation hitting some of Jackson's notes. Sure, there was plenty of desperation in Jackson's voice on the original, but he appeared to be doing it on purpose, with a whisper-singing bite that really gave the track its attitude.

For as implausible as it seems now, there was once a time when Michael Jackson almost had a tough guy image. During the Thriller era he was hanging with zombies, knife fighting in subways, and dealing with baby's mama drama just like Biggie. By contrast, Fall Out Boy are probably best known for bassist Pete Wentz's fashion line and his marrying Ashlee Simpson, as well as egregiously lengthy song titles like "Our Lawyer Made Us Change The Name Of This Song So We Wouldn't Get Sued". None of these things, incidentally, will give you the air of, say, Johnny Cash. In fact, this may be the first instance—ever—in which Michael Jackson could've hypothetically beat up a punk band. Not a good sign.

On the original "Beat It", Jackson and co-producer Quincy Jones enlisted guitar hero Eddie Van Halen for a flashy solo. John Mayer, a virtuosic guitarist in his own right, takes the reigns for the Fall Out Boy remake. But Mayer's solo falls flat: what could've been a bold reinvention instead ends up as merely Van Halen-lite.

The final tally? Nothing more than a signature pop tune watered down by two of today's most fashionable musical acts.

5. Sheryl Crow - "Sweet Child o' Mine"
By all accounts, the birth of "Sweet Child o' Mine" was a shock to Guns N' Roses, as if the band suddenly stumbled upon the ability to write radio-ready pop songs. The tune paired G N' R's signature hard edge with a surprising accessibility: that iconic opening guitar lick alone is capable of implanting itself in one's brain for days on end.

But if the original tune succeeded almost by accident, then Sheryl Crow's take on "Sweet Child" fails for the opposite reason: it sounds exactly like something she would've written. With its toothless, middle-of-the-road acoustic arrangement, it's not that Crow makes the song too pretty; it's that she sounds like she really doesn't give a damn. All the passionate bite of the original is gone, and in its place, violins and countrified guitar jangle agreeably hash things out like two vegan vendors at the Lilith Fair.

The original "Where do we go?" breakdown is a dramatic exercise, starring Axl's vocal snarl, Steve Adler's gigantic drums, and Slash's wah-wah pedal. But in Crow's hands the song merely breaks down, careening off a cliff in a wailing mess of vocals, strings, and accordion.

4. Guns N' Roses - "Knockin' On Heaven's Door"
Some covers just leave you with more questions than answers, and this Bob Dylan homage from glam metal gods Guns N' Roses is one such an enigma. Exactly how much coke went up the hatch before covering Dylan sounded like a great idea to the Sunset Strip boys? Dylan's weary, reflective original fades out after two and a half minutes—did G N' R actually think the song could be improved upon with three more minutes of Axl's gratuitous hey-hey-yeahs, multiple wanky blues solos from Slash, and—oh, what the hell—a black gospel choir?

Dylan wrote "Knockin'" to score the nuanced Sam Peckinpaugh western Pat Garrett and Billy the Kid. Fittingly, the Guns N' Roses cover first surfaced in Jerry Bruckheimer's brainless NASCAR vehicle Days of Thunder, both movie and cover full of sound and fury, signifying nothing.

3. Britney Spears - "I Love Rock & Roll"
Lesser artists can more easily be forgiven for the ill-chosen cover song. But when a heavy hitter like Madonna (Don McLean's "American Pie") or Dolly Parton (Led Zeppelin's "Stairway to Heaven") releases such an auditory abomination, the offense is somehow magnified by their pop prominence. And so we have it in the case of Britney Spears and her disastrous take on "I Love Rock & Roll", most popularly (and properly) performed by Joan Jett & The Blackhearts.

It's quite apparent with this cover that Spears came to bury rock & roll, not to praise it. Whether neutering the obligatory guitar solo or adding third-rate record scratches to the mix, Britney and producer Rodney "Darkchild" Jerkins reveal nothing but a profound hatred of the genre. Their re-imagining succeeds only in making Joan Jett's version (and the original, penned by The Arrows) seem more vibrant and authentic by comparison. While Jett's cover pulses with undeniable rock & roll swagger, Britney's swings limply like half-hearted karaoke.

2. Scissor Sisters - "Comfortably Numb"
If nothing else, covers grab your attention. So it was a rather brilliant career move for the then-unknown Scissor Sisters to whip up a disco-friendly take on Pink Floyd's "Comfortably Numb" and release it as a single in 2004. And the result? To quote Barry Gibb: "Tragedy!"

Much like cramming Beethoven's Fifth into a cell ringtone, the Scissor Sisters dispense of everything that made the epic Pink Floyd original so great, from the poignant string arrangement to David Gilmour's expressive guitar soloing. Instead, we get Bee Gees-aping falsetto from frontman Jake Shears, mercilessly bland synth lines, and a guitar delay intro which, depending on your taste in classic rock, robs either Stevie Nicks' "Edge of Seventeen" or Pink Floyd's own "Run Like Hell". It's enough to incite another Disco Demolition Night.

Pink Floyd's Roger Waters once told Rolling Stone that he based the song lyrics for "Comfortably Numb" on a time he was injected with tranquilizers to treat stomach cramps. Repeated exposure to the Scissor Sisters cover produces similar effects.

1. Celine Dion - "You Shook Me All Night Long"
Like a "Warning: Attack Dogs" sign, the tagline of "Celine Dion playing air guitar" is all the advisement one should need to stay far, far away.* If for some reason that doesn't suffice, imagine her aero-strumming the opening chords to AC/DC's "You Shook Me All Night Long" on a live performance of VH-1's Divas Las Vegas in 2002. I can't in good conscience provide you a link to the YouTube clip, but if you have a penchant for icepick-in-the-ear masochism, knock yourself out.

Simply said, there's no means of attaching any value to Dion's cover of this fairly immortal rock song. It cannot even be appreciated in an ironic or kitschy sense, since—ostensibly at least—Dion has the capacity for neither. Instead, the Canadian pop queen performs a spinal tap on the jukebox classic, sucking all the dirty rock'n' roll fun out with a surgical precision.

* In the future, I recommend that the phrases "air guitar" and "Celine Dion" be legally required to stay more than 500 feet away from each other.