Judas Priest covering Fleetwood Mac's "The Green Manalishi" (one of those rare covers that completely trumps the original version in every way, IMHO)
Sonic Youth covering Delaney & Bonnie's "Superstar" (this version is so haunting, so dark, and so creepy, it really brings to the surface how inherently disturbing most love songs are, with their stalker-ish messages of "not being able to live without you" and "can't stop thinking of you" and yada yada yada, when you think about 'em for longer than 5 seconds, most traditional love songs are completely obsessive, psychotic rants that hinge the stability of the singer's sanity and the validity of his identity entirely on their romantic infatuation with the object of obsession... gah)
Saint Vitus covering Black Flag's "Thirsty & Miserable" (I'm so smitten by the idea of an ultra-slow doom metal band covering an ultra-fast hardcore band, and the result is so fantastic here, I think)
Rammstein covering The Ramones' "Pet Semetary" (jarring for me at first, but hearring Rammstein do it in their heavily accented style of English has become an ever-reliable way of putting a smile on my face, especially when that great singalong chorus hit)
The Cramps covering Little Willie John's "Fever" (used to great effect in the bar slaughter sequence of Near Dark)
Racer X cover Blue Oyster Cult's "Godzilla" (still can't hold a candle to the original, but still pretty memorable nonetheless, great shredding from this band)
Slayer covering Iron Butterfly's "Inna Gadda Da Vida" (holy crap! my favorite thrash band covering one of my favorite classic stoner rock/proto-metal songs? and making it into a terrifying maelstrom that sounds like Satan hisself leading me down into the infernal pit?!? HELL YEAH! ...and they leave out that annoyingly endless drum & organ solo that goes on for what seems like a half a freaking hour? THANK BEELZEBUB!)
Nick Cave covering the traditional murder ballad "Stagger Lee" (debateable whether or not this should be considered a cover in the stricted sense, as this is an age-old folk song that has been re-imaged numerous times before, but this particular version, from Nick Cave'sMurder Ballads album, my favorite of his records, is just fantastic... as is every other song on the album, any one of which could easily be snuck onto this list)
Johnny Cash covering Nick Cave's "The Mercy Seat" (what's good for the goose is goo for the gander, and Nick Cave's own work got a great cover courtesy of the master himself, Johnny Cash... not as well-known as "Hurt" or "God's Gonna Cut You Down" but just as good if you ask me)
Echo & The Bunnymen covering The Doors' "People Are Strange" (I heard this version years before I everh eard the original version, thanks mainly to the opening credits of The Lost Boys, to this day it still remains my preferred rendition)
Celtic Frost covering Wall Of Voodoo's "Mexican Radio" (Celtic Frost caught a lot of s**t for this rather unconventional cover choice but I think it's great, just weird enough to be fun and still genuinely enjoyable as a song... has enough of the original song to make me want to sing along, but enough metal to make me want to bang my head)
Emily Browning covering Eurythmics' "Sweet Dreams" (I was genuinely surprised when I found out it was Emily Browning singing this, and I was even more surprised by how much I liked her rendition)
Finally, that brings us to my vote for the single best cover of all time...
SOD covering Jimi Hendrix's "Purple Haze" (rechristened as "The Ballad Of Jimi Hendrix")
Honrable mentions: Bathory - Black Diamond, Marilyn Manson - Sweet Dreams, Xentrix - Ghostbusters, Johnny Cash - Hurt, Johnny Cash - God's Gonna Cut You Down, Johnny Cash - Ghost Riders In The Sky, White Wizzard - Heading Out To The Highway, Metallica - So What?, Reverend Bizarre - The Gates Of Nanna, every other murder ballad on Nick Cave's Murder Ballads album, Jimi Hendri - All Along The Watchtower, Joey Ramone - What A Wonderful World, Bob Dylan (sort of) - Friday (you know you love it), AFI - Last Caress, Placebo - Running Up That Hill, Black Sabbath - Evil Woman, Ozzy Osbourne - 21st Century Schizoid Man, Judas Priest - Diamonds & Rust, Smith - Baby It's You, Demons & Wizards - White Room, the rest of the SOD Ballads
I love cover songs.