I'll post (with alterations) what I posted in the original thread I made on another forum that Schmungkins nicked this idea off of.
1: Dimmu Borgir-Burn in Hell (Cover of Twisted Sister)
I can imagine this song making black metal purists the world over cringe. Dimmu Borgir inject massive amounts of what can only be described as gleeful evil into this cheesy hair metal track. The pure genius touch is the way the first voice, which is supposed to be Satan, clears his throat delicately before screaming 'YOU'RE GONNA BURN IN HELL!'
2: Cradle of Filth-Death Comes Ripping (Cover of the Misfits)
Cradle of Filth managed to turn this rather light (despite it's title) horrorpunk track into what is easily their most vicious and extreme track. Dani's vocals are a constant bone-saw screech, accompanied by furious drumming and riffage that almost perfectly fuses punk and black metal.
3: Graveworm-Losing my Religion (Cover of REM)
I don't quite know what they did to this track, yet it's both unregocnisable and totally amazing.
4: Amorphis-Light my Fire (Cover of the Doors)
Coming as the bonus track at the end of an exceptionally bleak and low-key album based on all the most depressing bits of Finnish folklore, the up-beat guitar and cheesy keyboard (still set to organ but tuned up several octaves) come as a massive system shock. The best thing about this is the fact that the singing is still in an insanely low growl, with freuqent death metal grunts.
5: Arch Enemy-Aces High (Cover of Iron Maiden)
Arch Enemy have guitar that could blow almost any other band on the planet out the water: some of their old riffs and solos are unmatchable. They competently tackle this already amazing maiden song, and with the addition of Johan's death vocals, the song becomes just something else.
6: Therion-Summernight City (Cover of Abba)
I simply could not believe this had originally been an Abba track till I listened to the original. The effect they've created, fully utlising their normal tricks of choirs, multiple solo vocalists, layered electric guitar riffs and a full orchestra turn it into almost an opera piece.
7: Behemoth-Wish (Cover of Nine Inch Nails)
Behemoth saw the annoying electronic edges off this NIN track with pure metallic aggression, and hard-edged vocals that sound synthesised without the need for any machinery, giving the song an excellent atmosphere.
8: Anal Cunt-Staying Alive (Cover of the Beegees)
'OI! OI! OI! OI! STAYING ALOIVE!' Anal Cunts ultraviolent punk cover of this blandly inoffensive song has never failed to make anyone who's heard it laugh, making it possibly one of the funniest songs ever, and complete justification for the bands existence even on it's own.
9: Inkubus Sukkubus-Paint it Black (Cover of the Rolling Stones)
Inkubus Sukkubus bring just the right amount of anguish and synthesised goth rock drumming to remove any and all cheer this song ever had.
10: Apocalyptica-Master of Puppets (Cover of Metallica)
It's Master of Puppets played by a string quartet. 'nuff said.
Honourable mentions:
All the other Apocalyptica covers, (especially their cover of Ulvers 'Of Wolf and Man')
Johnny Cash-Hurt (Nine Inch Nails)
Children of Bodom-Somebody put Something in my Drink (The Ramones)
Type O Negative-Back in the USSR (The Beatles)