Best Cover Songs....

I love Judas Priest's cover of Johnny B Goode. Their albums from the era were pretty crap (Ram It Down, Turbo) but that cover is great.
Iron Maiden's covers are amazing too, like I've Got The Fire and their cover of Jethro Tull's Cross Eyed Mary.
 
Gamma Ray - Long live rock 'n roll
Eddy Antonini - I want out
Axel Rudi Pell - When a blind man cries
Slayer - In the garden of Eden (I'm not sure if that's the name of the song, it's from Iron Butterfly, this cover kicks ass)
 
Slayer's cover of Priest's Dissident Aggressor is fucking superb. Their covers album was shit though.
Some of G N'Rs stuff on The Spaghetti Incident is great too (Down on the farm, Black Leather etc.)
 
Opeth and Obituary both do awesome covers of Celtic Frost's The Circle Of The Tyrants. Death did an interesting cover of Judas Priest's Painkiller. Monstrosity did a cool cover of Angel Of Death. Immortal did a cool cover of Celtic Frost's Beyond The North Waves.
 
Ripfence Desperate Cry....................................................oh, how about Cannibal Corpses Metallica cover, fuck me if I can't remember what it's called....Four Horsemen? Anyway, it shreds.
 
That Iron Butterfly song is called 'in-agadda-da-vida' or something like that.

I'm not saying this is like one of the best covers, but I really like Cradle of Filth's version of Hallowed Be Thy Name.

:headbang:
 
A few that spring to mind....

Brutal Truth - Lord of This World
Me First & the Gimme Gimmes - Country Roads
Acid Reign - Hanging on the Telephone
Spoiler - You're the One That I Want