Best Cover Songs....

Agalloch - Kneel to the Cross (Sol Invictus)
My Dying Bride - Roads (Portishead)
My Dying Bride - Some Velvet Morning (Lee Hazlewood / Nancy Sinatra)
Novembre - Cloudbursting (Kate Bush)
Faith No More - War Pigs (Black Sabbath)
Faith No More - Easy (Lionel Richie)
Faith No More - I Started a Joke (The Beegees)
Leæther Strip - Sex Dwarf (Soft Cell)
Failure - Enjoy the Silence (Depeche Mode)
Evergreen Terrace - Enjoy the Silence (Depeche Mode)
Evergreen Terrace - Sunday Bloody Sunday (U2)
Cave In - Magnified (Failure)
Cave In - Dazed and Confused (Led Zeppelin)
 
Callenish Circle - Out Of The Body (Pestilence)
Death - Painkiller (Judas Priest)
Cradle Of Filth - Hallowed Be Thy Name (Iron Maiden)
Eternal Tears Of Sorrow - Flight Of Icarus (Iron Maiden)
Eternal Tears Of Sorrow - Black Tears (Edge Of Sanity)
Opeth - Remember Tomorrow (Iron Maiden)
Metallica - Blitzkrieg (Blitzkrieg)
Metallica - Am I Evil? (Diamondhead)
Agalloch - Kneel To The Cross (Sol Invictus)
Slayer - Dissident Agressor (Judas Priest)
Iced Earth - Shooting Star (Bad Company)
Cannibal Corpse - Zero The Hero (Black Sabbath)
Graveworm - Fear Of The Dark (Iron Maiden)
 
My favorites include...

Black Sabbath - Warning (Aynsly Dunbar's Retalliation)
Metallica - Am I Evil (Diamond Head), Breadfan (Budgie), Sabbra Caddabra (Black Sabbath), Blitzkrieg (Blitzkrieg), The Small Hours (Holocaust), the Mercyful Fate medley, Astronomy (Blue Oyster Cult)
Rainbow - Still I'm Sad (The Yardbirds)
Cathedral - Solitude (Black Sabbath)
Candlemass - Black Sabbath Medley
Bruce Dickinson - The Zoo (Scorpions)
Shadows Fall - Welcome to the Machine (Pink Floyd)
Jimi Hendrix - Hey Joe (Billy Roberts), All Along the Watchtower (Bob Dillan)
Led Zeppelin - You Shook Me (Willie Dixon)
The Doors - Crawling King Snake (John Lee Hooker)
Yngwie Malmsteen & Ronnie James Dio - Dream On (Aerosmith)
Creed(!) - Riders on the Storm (The Doors)
Sepultura - Symptom of the Universe (Black Sabbath)
Corrosion of Conformity - Lord of This World (Black Sabbath)
Witchery - Fast As a Shark (Accept)
Slayer - Dissident Aggressor (Judas Priest)
Tony Iommi - Shakin' My Wings (Jethro Tull... original actually called 'To Cry You a Song')
Helloween - Locomotive Breath (Jethro Tull)
Fates Warning - Sign of the Southern Cross (Black Sabbath)
Pantera - Planet Caravan, Electric Funeral, Hole in the Sky (Black Sabbath)
 
In Flames - Murders In The Rue Morgue (Maiden)
In Flames - Eye Of The Beholder (Metallica)
Megadeth - Paranoid (Black Sabbath)
Sepultura - Symptom Of The Universe (Black Sabbath)
Motley Crue - Smokin' In The Boys Room (Cant Remember Band)
 
Jimi Hendrix - All Along the Watchtower (Bob Dylan)
Metallica - Die, Die My Darling (Misfits)
Johnny Cash - Hurt (Nine Inch Nails)
Slayer - In-A-Gadda-Da-Vida (Slayer)
Pantera - Cat Scratch Fever (Ted Nugent)

Those are my top 5 in no particular order.
 
A lot of you are mentioning Obituary's cover of Celtic Frost's Circle Of Tyrants but you all heard their Venom cover Buried Alive? It's pretty good.
 
Nevermore - The Sound of Silence (Simon & Garfunkel)
Paradise Lost - Xavier (Dead Can Dance)
Vintersorg - Rainbow Demon (Uriah Heep)
In Flames - Everything Counts (Depeche Mode)
 
Anthrax-Got The Time(Joe Jackson)
Metallica-Am I Evil?(Diamond Head)
Bruce Dickinson-All The Young Dudes(Mott The Hoople)
Queensryche-Scarbourough Fair(Simon And Garfunkel)
Van Halen-You Really Got Me Now(The Kinks)
Slayer-Dissident Aggressor(Judas Priest)
Exodus-Lowrider(War)
Death-Painkiller(Judas Priest)
Overkill-Tyrant(Judas Priest)
Iron Maiden-Cross-Eyed Mary(Jethro Tull)
Opeth-Circle Of The Tyrants(Celtic Frost)