Simply put, what are your favorite covers of songs you know (or don't know in some cases). Non-VGM, of course.

Sonata Arctica (covering Die With Your Boots on, by Iron Maiden)

Queensryche (covering Synchronicity II, by the Police)

Quiet Riot (covering Cum on Feel the Noize, by Slade)

Motley Crue (covering Paranoid by Black Sabbath and Anarchy in the UK by Sex Pistols)

Dream Theater (covering Master of Puppets by Metallica)

Poison (covering Suffrage City by David Bowie, Dead Flowers by the Rolling Stones and What I Like About You by the Romantics)

Judas Priest (covering/metalizing Johnny B. Goode)

I tend to use these versions more, either because I like them more or because they're fresher and I've listened to the originals to death.

Oh Geez! Man, got a ton for this topic, here I go:

At Vance: Winner Takes it All (originally by Abba)
Dream Evil: My Number One (originally by Helena Paparizou)
Freedom Call: Dancing with Tears in my Eyes (originally by Ultravox)
Gamma Ray: Gamma Ray (originally by Birthcontrol)
Gamma Ray: It's a Sin (originally by Pet Shop Boys)
HammerFall: Child of the Damned (originally by Warlord)
HammerFall: Rising Force (orignally by Yngwie Malmsteen)
Heavenly: When the Rain Begins to Fall (originally by Jermaine Jackson, Pia Zadora)
Helloween: The Helliion, Electric Eye (originally by Judas Priest)
Iron Savior: Desert Plains (originally by Judas Priest)
Labÿrinth: Feel ~Legend B. Remix~ (originally by Zenith X)
Labÿrinth: I Feel You/Ti Sento (an Italian pop song)
Labÿrinth: Die for My Sins (originally by Sanctuary)
Nightwish: The Phantom of the Opera (from Phantom of the Opera)
Nightwish: Over the Hills and Far Away (originally by Gary Moore)
Sonata Arctica: World in my Eyes (originally by Depeche Mode)
Stratovarius: Bloodstone (originally by Judas Priest)
Stratovarius: Kill the King (originally by Rainbow)
Stratovarius: I Surrender (originally by Rainbow)
Vision Divine: The Needle Lies (originally by Queensryche)
Vision Divine: Take on Me (originally by Ah-Ha)
Vision Divine: The Final Countdown (originally by Europe)
Warmen: Alone (originally by Heart)

I perfer many of these over the originals...

This isn't too bad but Sonata is about 50/50 when they cover something: I can't stand their cover of Fade to Black and they turned Helloween's "I Want Out" into a convoluted synth fest, which in this case silence would not be a crime. Freedom Call's take on Dr. Stein is pretty terrible as well.

"She's Not There" by Santana.

Good classic Santana goodness, made a little better from The Zombies' original.

Antimatter - "Black Sun" (Original by Dead Can Dance)