I usually like covers that sound nothing like the originals. Some of my favorites :
Mercenary - Music Non Stop (pop singer unknown of me and I'm too lazy to search his/her name on google right now
)
Evergrey - I'm Sorry (same as above, I don't know the original artist)
Gamma Ray - Heavy Metal Mania (Holocaust, much better than the original IMO)
Thin Lizzy - Whiskey in the Jar
Angra - Kashmyr (Led Zeppelin, different arrangements)
Sanctuary - White Rabbit (Jefferson Airplane)
Scanner - Innuendo (Queen)
Sepultura - Orgasmatron (Motorhead)
Death - Painkiller (Judas Priest)
Killers - L'Aigle Noir (Barbara, a french singer of the 60's/70's, maybe the most surprising metal cover I've ever heard)
Judas Priest - Green Manalishi (Fleetwood Mac)
Judas Priest - Diamonds and Rust (Joan Baez)
Witchery - Restless and Wild (Accept)
Anthrax - Antisocial (Trust)
Manowar - Nessun Dorma (Puccini, Eric Adams is the man here)
Saxon - In the Court of the Crimson King (King Crimson, very cool heavy version of the KC classic)
Heaven's Gate - Always Look at the Bright Side of Life (from the Monthys Python, love that one!)
Vicious Rumors - Paint it Black (Rolling Stones, furious cover)
Rage - Paint in Black (Rolling Stones, just perfect!)
Hammerfall - Man on the Silver Mountain (Rainbow)
Angel Dust - Temple of the King (Rainbow)
Slayer - I Hate You (Verbal Abuse)
Deliverance - Sanctuary (?)
The Traceelords - Born to Be Alive (ahahah
)